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1. Shard Calculation

Query:

Response:

Calculated Shard: 179 (from laksa102)

2. Crawled Status Check

Query:

curl -X POST \
  'http://laksa179.int.ahrefs:8124/' \
  -H 'Content-Type: text/plain' \
  -H 'X-ClickHouse-Database: crawler3' \
  -H 'Authorization: Basic YXBpOg==' \
  -d 'SELECT getAhrefsURLFromUnparsed(src_unparsed) AS found_url, ifNull(toUnixTimestamp(download_stamp), 0) AS crawl_time, ifNull(toUnixTimestamp(props_url_first_seen), 0) AS first_indexed_time, download_http_code AS http_code, src_unparsed AS src_unparsed, src_root_hash AS src_root_hash, history_drop_reason AS history_drop_reason, meta_title AS meta_title, meta_descriptions AS meta_descriptions, attrs_boilerpipe_text AS attrs_boilerpipe_text, attrs_markdown AS attrs_markdown, attrs_readable_markdown AS attrs_readable_markdown, meta_canonical AS meta_canonical FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://publichealth.jhu.edu/2024/why-covid-cases-rise-every-summer\')), getAhrefsUnparsedNoserviceFromURL(\'https://publichealth.jhu.edu/2024/why-covid-cases-rise-every-summer\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/publichealth.jhu.edu\/2024\/why-covid-cases-rise-every-summer","crawl_time":1776729330,"first_indexed_time":1723123129,"http_code":200,"src_unparsed":"edu,jhu!publichealth,\/2024\/why-covid-cases-rise-every-summer s443","src_root_hash":"6599457417117746179","history_drop_reason":null,"meta_title":"Why COVID Cases Rise Every Summer | Johns Hopkins | Bloomberg School of Public Health","meta_descriptions":["Hot weather, human behavior patterns, and an easily mutating virus create the perfect recipe for COVID to peak in the summer."],"attrs_boilerpipe_text":"Fall and winter are known as the time when respiratory viruses surge. When COVID emerged in 2020, it joined flu and RSV as one of the common respiratory viruses that peaks during the colder months. Since then, COVID has transitioned from pandemic to endemic, but has also maintained dual seasonality, peaking twice a year.\nEvery summer since 2020, COVID rates have risen in July and August, due to a confluence of virological, behavioral, and environmental factors. While it’s still too early to say whether this dual seasonality will peter out or become the norm, understanding why these summer surges happen can help us better protect against infection, severe disease, and ruined vacations.\nWhy do we see waves of COVID infections in the summer?\nSeveral factors drive summer COVID waves. By mid-to-late summer, many people’s immunity—either from their last vaccination in the fall or from a previous infection—has waned considerably. That, combined with the emergence of \nmore transmissible variants\n, makes the chance of infection more likely.\nHuman behavior also plays a major role. As the weather gets hot, we spend more time in air-conditioned indoor spaces, where most virus transmission occurs, explains \nAndy Pekosz\n, PhD, a professor in \nMolecular Microbiology and Immunology\n. With the windows closed to keep the cool air inside, we restrict ventilation and air circulation that has shown to reduce virus spread.\nMany people travel more during the summer, with roughly half of Americans surveyed saying they planned to \ntravel more, farther, and for longer in 2024\n(link is external)\nthan the year prior. Travelers are not only exposed to more people—including from regions with more COVID cases—but also may be more likely to write off mild symptoms as simply the result of jet lag, not illness. Tiredness, headache, or a sore throat—all common after a long flight—are also symptoms of COVID, and without testing to be sure, a traveler may unknowingly expose others.\nPekosz experienced this himself after an international trip in July. At first he chalked up feeling tired to the massive time change, but when he started sneezing, he tested at home and it was positive. “I could have easily blown that off and just attributed everything to jet lag,” he says. “With a mildly symptomatic case, I may not have registered that it could be COVID and gone about my business for another couple of days while I was infectious.”\nWhy are there summer waves of COVID infections but not other respiratory illnesses like flu and RSV?\nCOVID’s ongoing dual seasonality can be attributed to both the higher presence of the virus year-round and its ability to mutate.\n“The number of COVID cases that are present year-round is so much higher than we see for flu or RSV,” Pekosz explains. With that higher baseline, when conditions make for easier transmission—like increased travel and heat driving people indoors—there are more cases to fuel a larger wave of infections. “With flu or RSV, there are so few baseline cases during the summer that these changes in behavior don’t cause the same spread.”\nEven when COVID rates are relatively low, more cases circulating year-round means more opportunities for the virus to mutate, which SARS-CoV-2 has proven to do very easily. When this happens, new variants emerge that are often more transmissible and better able to evade immunity. “The virus has shown itself to be very malleable, very able to handle mutations,” Pekosz says.\nHow might summer COVID waves impact our vaccination strategies?\nThe 2023-24 respiratory virus season marked a shift in COVID vaccination strategy in the U.S. to one of \nannual fall vaccination\n. That means that—just as with annual flu vaccines—governing bodies like the FDA and CDC use global surveillance data to predict which strains will be circulating and recommend a corresponding vaccine formulation. This happens in the spring so that manufacturers can prepare a vaccine for distribution in early fall.\nBut when infection rates increase, so do the chances for the virus to mutate. A wave of infections in the summer could result in new dominant variants that the vaccine may not protect against as effectively.\nWe saw this evolution play out in real time earlier this year. The FDA’s vaccine advisory panel was scheduled to meet in mid-May to make their recommendations for the fall, but they postponed the meeting to early June to allow more time to monitor the \nFLiRT variants\nthat had recently become dominant in the U.S. On June 6, the FDA recommended monovalent \nJN.1\n-lineage vaccines, and then a week later \nupdated their recommendations to a preference for the KP.2 strain\n(link is external)\n, a member of the JN.1 family of variants.\n“From a public health perspective, it would be great if we could send the same ‘go get your vaccines before respiratory season’ message at one time of year, with the updated vaccines timed to be available then,” Pekosz says. But consistent summer COVID waves could impact those recommendations. “It may end up that some individuals, particularly high risk individuals, are going to be urged to get vaccinated in early summer ahead of a presumed summer wave,” he says.\nHowever, it’s unlikely that the upcoming season’s updated vaccines would be available by then, and those individuals would only have access to the previous season’s formulation. The previous season’s formulation would still provide some protection against severe disease and hospitalization, but it likely would have reduced effectiveness against newer variants.\nWill COVID ever become seasonal like flu and RSV?\nWhile the U.S. has experienced a wave of COVID cases every summer since 2020, it’s too early to say whether this is a long-term trend. Many public health experts still expect the virus’s annual spread will eventually look more like those of flu and RSV, which tend to peak during the winter. But exactly how long it will take to settle into that seasonal pattern is not clear.\n“Historically, there are examples of viruses that do settle into a seasonal pattern after a transition period,” Pekosz says, pointing to the 1918 influenza pandemic, which he says took five or six years to take on the seasonal pattern we’ve come to expect. “We've got to hope that it'll settle into a more seasonal pattern, but we should also start thinking about strategies to deal with it if it doesn't.”\nCan we expect COVID cases to drop before they rise again in the winter?\nAccording to the \nCDC’s wastewater surveillance\n(link is external)\n, COVID activity is high or very high in many states, with levels in the South and West \nnearing what they were in January\n(link is external)\n. As students and teachers head back to school over the next few weeks, Pekosz expects cases will continue to spread before the fall vaccine formulation is rolled out.\nWhile infection-induced immunity comes with higher risks than vaccine-induced immunity, this late summer wave will offer good population-level protection that will allow for case rates to trend back down. “I expect we’ll see a little bit of a lull starting around the middle of September or so, before we start seeing the late winter surge of COVID cases,” Pekosz says.\nAliza Rosen is a digital content strategist in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health.\nRELATED:\nWhat to Know About COVID FLiRT Variants\nUnderstanding the CDC’s Updated COVID Isolation Guidance\nMore Americans Could Benefit from Paxlovid for COVID Infection","attrs_markdown":"[Johns Hopkins UniversityEst. 1876 America’s First Research University](https:\/\/150.jhu.edu\/)\n\n[Skip to main content](https:\/\/publichealth.jhu.edu\/2024\/why-covid-cases-rise-every-summer#main-content)\n\n- [Expand](https:\/\/publichealth.jhu.edu\/2024\/why-covid-cases-rise-every-summer) [Departments](https:\/\/publichealth.jhu.edu\/departments)\n  - 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[At a Glance](https:\/\/publichealth.jhu.edu\/about\/at-a-glance)\n  - [Leadership](https:\/\/publichealth.jhu.edu\/about\/leadership)\n  - [Student Life](https:\/\/publichealth.jhu.edu\/about\/student-life)\n  - [Campus](https:\/\/publichealth.jhu.edu\/about\/our-campus)\n  - [Baltimore](https:\/\/publichealth.jhu.edu\/about\/our-home-city-of-baltimore)\n  - [Inclusion, Diversity, Anti-Racism, and Equity (IDARE)](https:\/\/publichealth.jhu.edu\/about\/inclusion-diversity-anti-racism-and-equity-idare)\n  - [History](https:\/\/publichealth.jhu.edu\/about\/history)\n  - [What is Public Health?](https:\/\/publichealth.jhu.edu\/about\/what-is-public-health)\n- [Apply](https:\/\/publichealth.jhu.edu\/offices-and-services\/office-of-admissions-services\/how-to-apply)\n\n- [Expand](https:\/\/publichealth.jhu.edu\/2024\/why-covid-cases-rise-every-summer) [Departments](https:\/\/publichealth.jhu.edu\/departments)\n  - [Biochemistry and Molecular Biology](https:\/\/publichealth.jhu.edu\/departments\/biochemistry-and-molecular-biology)\n  - [Biostatistics](https:\/\/publichealth.jhu.edu\/departments\/biostatistics)\n  - [Environmental Health and Engineering](https:\/\/publichealth.jhu.edu\/departments\/environmental-health-and-engineering)\n  - [Epidemiology](https:\/\/publichealth.jhu.edu\/departments\/epidemiology)\n  - [Health Policy and Management](https:\/\/publichealth.jhu.edu\/departments\/health-policy-and-management)\n  - [Health, Behavior and Society](https:\/\/publichealth.jhu.edu\/departments\/health-behavior-and-society)\n  - [International Health](https:\/\/publichealth.jhu.edu\/departments\/international-health)\n  - [Mental Health](https:\/\/publichealth.jhu.edu\/departments\/mental-health)\n  - [Molecular Microbiology and Immunology](https:\/\/publichealth.jhu.edu\/departments\/molecular-microbiology-and-immunology)\n  - [Population, Family and Reproductive Health](https:\/\/publichealth.jhu.edu\/departments\/population-family-and-reproductive-health)\n- [Faculty](https:\/\/publichealth.jhu.edu\/faculty\/directory\/list)\n- [Topics](https:\/\/publichealth.jhu.edu\/topics)\n\n- [Departments »](https:\/\/publichealth.jhu.edu\/departments)\n  - [Biochemistry and Molecular Biology](https:\/\/publichealth.jhu.edu\/departments\/biochemistry-and-molecular-biology)\n  - [Biostatistics](https:\/\/publichealth.jhu.edu\/departments\/biostatistics)\n  - [Environmental Health and Engineering](https:\/\/publichealth.jhu.edu\/departments\/environmental-health-and-engineering)\n  - [Epidemiology](https:\/\/publichealth.jhu.edu\/departments\/epidemiology)\n  - [Health Policy and Management](https:\/\/publichealth.jhu.edu\/departments\/health-policy-and-management)\n  - [Health, Behavior and Society](https:\/\/publichealth.jhu.edu\/departments\/health-behavior-and-society)\n  - [International Health](https:\/\/publichealth.jhu.edu\/departments\/international-health)\n  - [Mental Health](https:\/\/publichealth.jhu.edu\/departments\/mental-health)\n  - [Molecular Microbiology and Immunology](https:\/\/publichealth.jhu.edu\/departments\/molecular-microbiology-and-immunology)\n  - [Population, Family and Reproductive Health](https:\/\/publichealth.jhu.edu\/departments\/population-family-and-reproductive-health)\n- [Faculty](https:\/\/publichealth.jhu.edu\/faculty\/directory\/list)\n- [Topics](https:\/\/publichealth.jhu.edu\/topics)\n\nSearch\n\nBloomberg School\n\n# Why COVID Surges in the Summer\nHot weather, human behavior patterns, and an easily mutating virus create the perfect recipe for COVID’s peak in the summer.\n\nPublished\n\nAugust 07, 2024\n\nBy\n\nAliza Rosen\n\n[COVID-19](https:\/\/publichealth.jhu.edu\/topics\/covid-19-and-public-health)\n\n[Infectious Diseases](https:\/\/publichealth.jhu.edu\/topics\/infectious-diseases-and-public-health)\n\n[Vaccines](https:\/\/publichealth.jhu.edu\/topics\/vaccines-and-public-health)\n\n![An illustration of a wave in the ocean covered in COVID molecules with a person swimming underneath it holding a vaccine](https:\/\/publichealth.jhu.edu\/sites\/default\/files\/styles\/article_feature\/public\/2024-08\/summer-covid-wave.jpg?h=6e972868&itok=hReWAg8L)\n\nFall and winter are known as the time when respiratory viruses surge. When COVID emerged in 2020, it joined flu and RSV as one of the common respiratory viruses that peaks during the colder months. Since then, COVID has transitioned from pandemic to endemic, but has also maintained dual seasonality, peaking twice a year.\n\nEvery summer since 2020, COVID rates have risen in July and August, due to a confluence of virological, behavioral, and environmental factors. While it’s still too early to say whether this dual seasonality will peter out or become the norm, understanding why these summer surges happen can help us better protect against infection, severe disease, and ruined vacations.\n\n**Why do we see waves of COVID infections in the summer?**\n\nSeveral factors drive summer COVID waves. By mid-to-late summer, many people’s immunity—either from their last vaccination in the fall or from a previous infection—has waned considerably. That, combined with the emergence of [more transmissible variants](https:\/\/publichealth.jhu.edu\/2024\/what-to-know-about-covid-flirt-variants), makes the chance of infection more likely.\n\nHuman behavior also plays a major role. As the weather gets hot, we spend more time in air-conditioned indoor spaces, where most virus transmission occurs, explains [Andy Pekosz](https:\/\/publichealth.jhu.edu\/faculty\/1972\/andrew-stanley-pekosz), PhD, a professor in [Molecular Microbiology and Immunology](https:\/\/publichealth.jhu.edu\/departments\/molecular-microbiology-and-immunology). With the windows closed to keep the cool air inside, we restrict ventilation and air circulation that has shown to reduce virus spread.\n\nMany people travel more during the summer, with roughly half of Americans surveyed saying they planned to [travel more, farther, and for longer in 2024](https:\/\/www.prnewswire.com\/news-releases\/tripadvisor-predicts-busy-summer-travel-season-for-americans-an-increasing-desire-for-adventure-302144040.html \"(opens in a new window)\") than the year prior. Travelers are not only exposed to more people—including from regions with more COVID cases—but also may be more likely to write off mild symptoms as simply the result of jet lag, not illness. Tiredness, headache, or a sore throat—all common after a long flight—are also symptoms of COVID, and without testing to be sure, a traveler may unknowingly expose others.\n\nPekosz experienced this himself after an international trip in July. At first he chalked up feeling tired to the massive time change, but when he started sneezing, he tested at home and it was positive. “I could have easily blown that off and just attributed everything to jet lag,” he says. “With a mildly symptomatic case, I may not have registered that it could be COVID and gone about my business for another couple of days while I was infectious.”\n\n**Why are there summer waves of COVID infections but not other respiratory illnesses like flu and RSV?**\n\nCOVID’s ongoing dual seasonality can be attributed to both the higher presence of the virus year-round and its ability to mutate.\n\n“The number of COVID cases that are present year-round is so much higher than we see for flu or RSV,” Pekosz explains. With that higher baseline, when conditions make for easier transmission—like increased travel and heat driving people indoors—there are more cases to fuel a larger wave of infections. “With flu or RSV, there are so few baseline cases during the summer that these changes in behavior don’t cause the same spread.”\n\nEven when COVID rates are relatively low, more cases circulating year-round means more opportunities for the virus to mutate, which SARS-CoV-2 has proven to do very easily. When this happens, new variants emerge that are often more transmissible and better able to evade immunity. “The virus has shown itself to be very malleable, very able to handle mutations,” Pekosz says.\n\n**How might summer COVID waves impact our vaccination strategies?**\n\nThe 2023-24 respiratory virus season marked a shift in COVID vaccination strategy in the U.S. to one of [annual fall vaccination](https:\/\/publichealth.jhu.edu\/2023\/what-to-know-about-the-updated-covid-19-vaccine-for-fall\/winter-2023). That means that—just as with annual flu vaccines—governing bodies like the FDA and CDC use global surveillance data to predict which strains will be circulating and recommend a corresponding vaccine formulation. This happens in the spring so that manufacturers can prepare a vaccine for distribution in early fall.\n\nBut when infection rates increase, so do the chances for the virus to mutate. A wave of infections in the summer could result in new dominant variants that the vaccine may not protect against as effectively.\n\nWe saw this evolution play out in real time earlier this year. The FDA’s vaccine advisory panel was scheduled to meet in mid-May to make their recommendations for the fall, but they postponed the meeting to early June to allow more time to monitor the [FLiRT variants](https:\/\/publichealth.jhu.edu\/2024\/what-to-know-about-covid-flirt-variants) that had recently become dominant in the U.S. On June 6, the FDA recommended monovalent [JN.1](https:\/\/publichealth.jhu.edu\/2024\/jn1-the-dominant-variant-in-the-covid-surge)\\-lineage vaccines, and then a week later [updated their recommendations to a preference for the KP.2 strain](https:\/\/www.fda.gov\/vaccines-blood-biologics\/updated-covid-19-vaccines-use-united-states-beginning-fall-2024 \"(opens in a new window)\"), a member of the JN.1 family of variants.\n\n“From a public health perspective, it would be great if we could send the same ‘go get your vaccines before respiratory season’ message at one time of year, with the updated vaccines timed to be available then,” Pekosz says. But consistent summer COVID waves could impact those recommendations. “It may end up that some individuals, particularly high risk individuals, are going to be urged to get vaccinated in early summer ahead of a presumed summer wave,” he says.\n\nHowever, it’s unlikely that the upcoming season’s updated vaccines would be available by then, and those individuals would only have access to the previous season’s formulation. The previous season’s formulation would still provide some protection against severe disease and hospitalization, but it likely would have reduced effectiveness against newer variants.\n\n**Will COVID ever become seasonal like flu and RSV?**\n\nWhile the U.S. has experienced a wave of COVID cases every summer since 2020, it’s too early to say whether this is a long-term trend. Many public health experts still expect the virus’s annual spread will eventually look more like those of flu and RSV, which tend to peak during the winter. But exactly how long it will take to settle into that seasonal pattern is not clear.\n\n“Historically, there are examples of viruses that do settle into a seasonal pattern after a transition period,” Pekosz says, pointing to the 1918 influenza pandemic, which he says took five or six years to take on the seasonal pattern we’ve come to expect. “We've got to hope that it'll settle into a more seasonal pattern, but we should also start thinking about strategies to deal with it if it doesn't.”\n\n**Can we expect COVID cases to drop before they rise again in the winter?**\n\nAccording to the [CDC’s wastewater surveillance](https:\/\/www.cdc.gov\/nwss\/rv\/COVID19-currentlevels.html \"(opens in a new window)\"), COVID activity is high or very high in many states, with levels in the South and West [nearing what they were in January](https:\/\/www.cdc.gov\/nwss\/rv\/COVID19-nationaltrend.html \"(opens in a new window)\"). As students and teachers head back to school over the next few weeks, Pekosz expects cases will continue to spread before the fall vaccine formulation is rolled out.\n\nWhile infection-induced immunity comes with higher risks than vaccine-induced immunity, this late summer wave will offer good population-level protection that will allow for case rates to trend back down. “I expect we’ll see a little bit of a lull starting around the middle of September or so, before we start seeing the late winter surge of COVID cases,” Pekosz says.\n\n*Aliza Rosen is a digital content strategist in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health.*\n\n## **RELATED:**\n- [What to Know About COVID FLiRT Variants](https:\/\/publichealth.jhu.edu\/2024\/what-to-know-about-covid-flirt-variants)\n- [Understanding the CDC’s Updated COVID Isolation Guidance](https:\/\/publichealth.jhu.edu\/2024\/virologist-explains-the-cdcs-new-covid-isolation-guidance)\n- [More Americans Could Benefit from Paxlovid for COVID Infection](https:\/\/publichealth.jhu.edu\/2024\/why-more-people-should-be-prescribed-paxlovid-for-covid)\n\n## More from the Bloomberg School\n- [See latest headlines](https:\/\/publichealth.jhu.edu\/headlines)\n- [Learn more about the Department of Molecular Microbiology and Immunology](https:\/\/publichealth.jhu.edu\/departments\/molecular-microbiology-and-immunology)\n\n## Related Content\n[![Photo of girl smiling at school](https:\/\/publichealth.jhu.edu\/sites\/default\/files\/styles\/related_content\/public\/2026-04\/AdobeStock_310920057.jpeg?h=c6e9cc1f&itok=4lswcnyf)](https:\/\/publichealth.jhu.edu\/311\/2026\/school-vaccination-checks-to-improve-coverage)\n\n### [Closing Immunity Gaps Through School Vaccination Checks](https:\/\/publichealth.jhu.edu\/311\/2026\/school-vaccination-checks-to-improve-coverage)\nApril 16, 2026\n\n[![A patient fills out a form while her blood and saliva are screened for Hepatitis C and HIV in Spencer, West Virginia, on September 28, 2017.](https:\/\/publichealth.jhu.edu\/sites\/default\/files\/styles\/related_content\/public\/2026-04\/hep-c.jpg?h=4f4378cb&itok=zR0m9KTH)](https:\/\/publichealth.jhu.edu\/2026\/what-to-know-about-hepatitis-c)\n\n### [What to Know About Hepatitis C](https:\/\/publichealth.jhu.edu\/2026\/what-to-know-about-hepatitis-c)\nApril 10, 2026\n\n### [Why Review of the U.S.’s Measles Elimination Status Has Been Delayed](https:\/\/publichealth.jhu.edu\/2026\/why-review-of-the-uss-measles-elimination-status-has-been-delayed)\nApril 09, 2026\n\n### [Media Briefing: mRNA Vaccines](https:\/\/publichealth.jhu.edu\/2026\/media-briefing-mrna-vaccines)\nMarch 26, 2026\n\n[![Veterinarian seated in front of client with small dog on her lap lets dog sniff his hand while he speaks to the client.](https:\/\/publichealth.jhu.edu\/sites\/default\/files\/styles\/related_content\/public\/2026-02\/pet-vaccines-veterinarian.jpg?h=f321a05b&itok=pykOpVbz)](https:\/\/publichealth.jhu.edu\/2026\/vaccinating-your-dog-or-cat-can-also-protect-you)\n\n### [Why We Vaccinate Our Dogs and Cats](https:\/\/publichealth.jhu.edu\/2026\/vaccinating-your-dog-or-cat-can-also-protect-you)\nFebruary 27, 2026\n\n**Johns Hopkins Bloomberg School of Public Health**  \n [615 N. Wolfe Street, Baltimore, MD 21205](https:\/\/goo.gl\/maps\/p6Eowe2n6W2FuJ3k7 \"(opens in a new window)\")\n\n## Footer social\n- [LinkedIn](https:\/\/www.linkedin.com\/school\/johns-hopkins-bloomberg-school-of-public-health\/ \"(opens in a new window)\")\n- [Facebook](https:\/\/www.facebook.com\/JohnsHopkinsSPH \"(opens in a new window)\")\n- [Instagram](https:\/\/www.instagram.com\/johnshopkinssph\/ \"(opens in a new window)\")\n- [BlueSky](https:\/\/bsky.app\/profile\/johnshopkinssph.bsky.social \"(opens in a new window)\")\n- [X (Formerly Twitter)](https:\/\/twitter.com\/JohnsHopkinsSPH \"(opens in a new window)\")\n- [Youtube](https:\/\/www.youtube.com\/user\/JohnsHopkinsSPH \"(opens in a new window)\")\n\n## Footer menu primary\n- [Offices & Services](https:\/\/publichealth.jhu.edu\/offices-and-services)\n- [Alumni](https:\/\/publichealth.jhu.edu\/alumni)\n- [Giving](https:\/\/publichealth.jhu.edu\/giving)\n- [Calendars](https:\/\/publichealth.jhu.edu\/calendars-at-the-bloomberg-school)\n- [Careers](https:\/\/publichealth.jhu.edu\/careers \"Job Opportunities and Career Resources\")\n- [Contact Us](https:\/\/publichealth.jhu.edu\/contact-us)\n\n## Footer menu secondary\n- [Centers & Institutes Directory](https:\/\/publichealth.jhu.edu\/centers-and-institutes-directory)\n- [My.PublicHealth](https:\/\/my.publichealth.jhu.edu\/)\n- [My.JH](https:\/\/my.jh.edu\/myJH\/ \"(opens in a new window)\")\n- [CoursePlus](https:\/\/courseplus.jhu.edu\/)\n- [Student Information System](https:\/\/sis.jhu.edu\/)\n\n[Johns Hopkins University](https:\/\/www.jhu.edu\/)\n\n## Footer menu extra\n- [Web Policies](https:\/\/publichealth.jhu.edu\/web-policies)\n- [Accessibility](https:\/\/accessibility.jhu.edu\/)\n- [Website Help](https:\/\/webhelp.publichealth.jhu.edu\/support\/tickets\/new)\n\n## We use cookies on this site to enhance your user experience.\nBy clicking the Accept button, you agree to us doing so.\n\nLearn more\n\nAccept\n\nNo, thanks","attrs_readable_markdown":"Fall and winter are known as the time when respiratory viruses surge. When COVID emerged in 2020, it joined flu and RSV as one of the common respiratory viruses that peaks during the colder months. Since then, COVID has transitioned from pandemic to endemic, but has also maintained dual seasonality, peaking twice a year.\n\nEvery summer since 2020, COVID rates have risen in July and August, due to a confluence of virological, behavioral, and environmental factors. While it’s still too early to say whether this dual seasonality will peter out or become the norm, understanding why these summer surges happen can help us better protect against infection, severe disease, and ruined vacations.\n\n**Why do we see waves of COVID infections in the summer?**\n\nSeveral factors drive summer COVID waves. By mid-to-late summer, many people’s immunity—either from their last vaccination in the fall or from a previous infection—has waned considerably. That, combined with the emergence of [more transmissible variants](https:\/\/publichealth.jhu.edu\/2024\/what-to-know-about-covid-flirt-variants), makes the chance of infection more likely.\n\nHuman behavior also plays a major role. As the weather gets hot, we spend more time in air-conditioned indoor spaces, where most virus transmission occurs, explains [Andy Pekosz](https:\/\/publichealth.jhu.edu\/faculty\/1972\/andrew-stanley-pekosz), PhD, a professor in [Molecular Microbiology and Immunology](https:\/\/publichealth.jhu.edu\/departments\/molecular-microbiology-and-immunology). With the windows closed to keep the cool air inside, we restrict ventilation and air circulation that has shown to reduce virus spread.\n\nMany people travel more during the summer, with roughly half of Americans surveyed saying they planned to [travel more, farther, and for longer in 2024](https:\/\/www.prnewswire.com\/news-releases\/tripadvisor-predicts-busy-summer-travel-season-for-americans-an-increasing-desire-for-adventure-302144040.html \"(opens in a new window)\") than the year prior. Travelers are not only exposed to more people—including from regions with more COVID cases—but also may be more likely to write off mild symptoms as simply the result of jet lag, not illness. Tiredness, headache, or a sore throat—all common after a long flight—are also symptoms of COVID, and without testing to be sure, a traveler may unknowingly expose others.\n\nPekosz experienced this himself after an international trip in July. At first he chalked up feeling tired to the massive time change, but when he started sneezing, he tested at home and it was positive. “I could have easily blown that off and just attributed everything to jet lag,” he says. “With a mildly symptomatic case, I may not have registered that it could be COVID and gone about my business for another couple of days while I was infectious.”\n\n**Why are there summer waves of COVID infections but not other respiratory illnesses like flu and RSV?**\n\nCOVID’s ongoing dual seasonality can be attributed to both the higher presence of the virus year-round and its ability to mutate.\n\n“The number of COVID cases that are present year-round is so much higher than we see for flu or RSV,” Pekosz explains. With that higher baseline, when conditions make for easier transmission—like increased travel and heat driving people indoors—there are more cases to fuel a larger wave of infections. “With flu or RSV, there are so few baseline cases during the summer that these changes in behavior don’t cause the same spread.”\n\nEven when COVID rates are relatively low, more cases circulating year-round means more opportunities for the virus to mutate, which SARS-CoV-2 has proven to do very easily. When this happens, new variants emerge that are often more transmissible and better able to evade immunity. “The virus has shown itself to be very malleable, very able to handle mutations,” Pekosz says.\n\n**How might summer COVID waves impact our vaccination strategies?**\n\nThe 2023-24 respiratory virus season marked a shift in COVID vaccination strategy in the U.S. to one of [annual fall vaccination](https:\/\/publichealth.jhu.edu\/2023\/what-to-know-about-the-updated-covid-19-vaccine-for-fall\/winter-2023). That means that—just as with annual flu vaccines—governing bodies like the FDA and CDC use global surveillance data to predict which strains will be circulating and recommend a corresponding vaccine formulation. This happens in the spring so that manufacturers can prepare a vaccine for distribution in early fall.\n\nBut when infection rates increase, so do the chances for the virus to mutate. A wave of infections in the summer could result in new dominant variants that the vaccine may not protect against as effectively.\n\nWe saw this evolution play out in real time earlier this year. The FDA’s vaccine advisory panel was scheduled to meet in mid-May to make their recommendations for the fall, but they postponed the meeting to early June to allow more time to monitor the [FLiRT variants](https:\/\/publichealth.jhu.edu\/2024\/what-to-know-about-covid-flirt-variants) that had recently become dominant in the U.S. On June 6, the FDA recommended monovalent [JN.1](https:\/\/publichealth.jhu.edu\/2024\/jn1-the-dominant-variant-in-the-covid-surge)\\-lineage vaccines, and then a week later [updated their recommendations to a preference for the KP.2 strain](https:\/\/www.fda.gov\/vaccines-blood-biologics\/updated-covid-19-vaccines-use-united-states-beginning-fall-2024 \"(opens in a new window)\"), a member of the JN.1 family of variants.\n\n“From a public health perspective, it would be great if we could send the same ‘go get your vaccines before respiratory season’ message at one time of year, with the updated vaccines timed to be available then,” Pekosz says. But consistent summer COVID waves could impact those recommendations. “It may end up that some individuals, particularly high risk individuals, are going to be urged to get vaccinated in early summer ahead of a presumed summer wave,” he says.\n\nHowever, it’s unlikely that the upcoming season’s updated vaccines would be available by then, and those individuals would only have access to the previous season’s formulation. The previous season’s formulation would still provide some protection against severe disease and hospitalization, but it likely would have reduced effectiveness against newer variants.\n\n**Will COVID ever become seasonal like flu and RSV?**\n\nWhile the U.S. has experienced a wave of COVID cases every summer since 2020, it’s too early to say whether this is a long-term trend. Many public health experts still expect the virus’s annual spread will eventually look more like those of flu and RSV, which tend to peak during the winter. But exactly how long it will take to settle into that seasonal pattern is not clear.\n\n“Historically, there are examples of viruses that do settle into a seasonal pattern after a transition period,” Pekosz says, pointing to the 1918 influenza pandemic, which he says took five or six years to take on the seasonal pattern we’ve come to expect. “We've got to hope that it'll settle into a more seasonal pattern, but we should also start thinking about strategies to deal with it if it doesn't.”\n\n**Can we expect COVID cases to drop before they rise again in the winter?**\n\nAccording to the [CDC’s wastewater surveillance](https:\/\/www.cdc.gov\/nwss\/rv\/COVID19-currentlevels.html \"(opens in a new window)\"), COVID activity is high or very high in many states, with levels in the South and West [nearing what they were in January](https:\/\/www.cdc.gov\/nwss\/rv\/COVID19-nationaltrend.html \"(opens in a new window)\"). As students and teachers head back to school over the next few weeks, Pekosz expects cases will continue to spread before the fall vaccine formulation is rolled out.\n\nWhile infection-induced immunity comes with higher risks than vaccine-induced immunity, this late summer wave will offer good population-level protection that will allow for case rates to trend back down. “I expect we’ll see a little bit of a lull starting around the middle of September or so, before we start seeing the late winter surge of COVID cases,” Pekosz says.\n\n*Aliza Rosen is a digital content strategist in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health.*\n\n## **RELATED:**\n- [What to Know About COVID FLiRT Variants](https:\/\/publichealth.jhu.edu\/2024\/what-to-know-about-covid-flirt-variants)\n- [Understanding the CDC’s Updated COVID Isolation Guidance](https:\/\/publichealth.jhu.edu\/2024\/virologist-explains-the-cdcs-new-covid-isolation-guidance)\n- [More Americans Could Benefit from Paxlovid for COVID Infection](https:\/\/publichealth.jhu.edu\/2024\/why-more-people-should-be-prescribed-paxlovid-for-covid)","meta_canonical":null}

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Meta Title	Why COVID Cases Rise Every Summer \| Johns Hopkins \| Bloomberg School of Public Health
Meta Description	Hot weather, human behavior patterns, and an easily mutating virus create the perfect recipe for COVID to peak in the summer.
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Boilerpipe Text	Fall and winter are known as the time when respiratory viruses surge. When COVID emerged in 2020, it joined flu and RSV as one of the common respiratory viruses that peaks during the colder months. Since then, COVID has transitioned from pandemic to endemic, but has also maintained dual seasonality, peaking twice a year. Every summer since 2020, COVID rates have risen in July and August, due to a confluence of virological, behavioral, and environmental factors. While it’s still too early to say whether this dual seasonality will peter out or become the norm, understanding why these summer surges happen can help us better protect against infection, severe disease, and ruined vacations. Why do we see waves of COVID infections in the summer? Several factors drive summer COVID waves. By mid-to-late summer, many people’s immunity—either from their last vaccination in the fall or from a previous infection—has waned considerably. That, combined with the emergence of more transmissible variants , makes the chance of infection more likely. Human behavior also plays a major role. As the weather gets hot, we spend more time in air-conditioned indoor spaces, where most virus transmission occurs, explains Andy Pekosz , PhD, a professor in Molecular Microbiology and Immunology . With the windows closed to keep the cool air inside, we restrict ventilation and air circulation that has shown to reduce virus spread. Many people travel more during the summer, with roughly half of Americans surveyed saying they planned to travel more, farther, and for longer in 2024 (link is external) than the year prior. Travelers are not only exposed to more people—including from regions with more COVID cases—but also may be more likely to write off mild symptoms as simply the result of jet lag, not illness. Tiredness, headache, or a sore throat—all common after a long flight—are also symptoms of COVID, and without testing to be sure, a traveler may unknowingly expose others. Pekosz experienced this himself after an international trip in July. At first he chalked up feeling tired to the massive time change, but when he started sneezing, he tested at home and it was positive. “I could have easily blown that off and just attributed everything to jet lag,” he says. “With a mildly symptomatic case, I may not have registered that it could be COVID and gone about my business for another couple of days while I was infectious.” Why are there summer waves of COVID infections but not other respiratory illnesses like flu and RSV? COVID’s ongoing dual seasonality can be attributed to both the higher presence of the virus year-round and its ability to mutate. “The number of COVID cases that are present year-round is so much higher than we see for flu or RSV,” Pekosz explains. With that higher baseline, when conditions make for easier transmission—like increased travel and heat driving people indoors—there are more cases to fuel a larger wave of infections. “With flu or RSV, there are so few baseline cases during the summer that these changes in behavior don’t cause the same spread.” Even when COVID rates are relatively low, more cases circulating year-round means more opportunities for the virus to mutate, which SARS-CoV-2 has proven to do very easily. When this happens, new variants emerge that are often more transmissible and better able to evade immunity. “The virus has shown itself to be very malleable, very able to handle mutations,” Pekosz says. How might summer COVID waves impact our vaccination strategies? The 2023-24 respiratory virus season marked a shift in COVID vaccination strategy in the U.S. to one of annual fall vaccination . That means that—just as with annual flu vaccines—governing bodies like the FDA and CDC use global surveillance data to predict which strains will be circulating and recommend a corresponding vaccine formulation. This happens in the spring so that manufacturers can prepare a vaccine for distribution in early fall. But when infection rates increase, so do the chances for the virus to mutate. A wave of infections in the summer could result in new dominant variants that the vaccine may not protect against as effectively. We saw this evolution play out in real time earlier this year. The FDA’s vaccine advisory panel was scheduled to meet in mid-May to make their recommendations for the fall, but they postponed the meeting to early June to allow more time to monitor the FLiRT variants that had recently become dominant in the U.S. On June 6, the FDA recommended monovalent JN.1 -lineage vaccines, and then a week later updated their recommendations to a preference for the KP.2 strain (link is external) , a member of the JN.1 family of variants. “From a public health perspective, it would be great if we could send the same ‘go get your vaccines before respiratory season’ message at one time of year, with the updated vaccines timed to be available then,” Pekosz says. But consistent summer COVID waves could impact those recommendations. “It may end up that some individuals, particularly high risk individuals, are going to be urged to get vaccinated in early summer ahead of a presumed summer wave,” he says. However, it’s unlikely that the upcoming season’s updated vaccines would be available by then, and those individuals would only have access to the previous season’s formulation. The previous season’s formulation would still provide some protection against severe disease and hospitalization, but it likely would have reduced effectiveness against newer variants. Will COVID ever become seasonal like flu and RSV? While the U.S. has experienced a wave of COVID cases every summer since 2020, it’s too early to say whether this is a long-term trend. Many public health experts still expect the virus’s annual spread will eventually look more like those of flu and RSV, which tend to peak during the winter. But exactly how long it will take to settle into that seasonal pattern is not clear. “Historically, there are examples of viruses that do settle into a seasonal pattern after a transition period,” Pekosz says, pointing to the 1918 influenza pandemic, which he says took five or six years to take on the seasonal pattern we’ve come to expect. “We've got to hope that it'll settle into a more seasonal pattern, but we should also start thinking about strategies to deal with it if it doesn't.” Can we expect COVID cases to drop before they rise again in the winter? According to the CDC’s wastewater surveillance (link is external) , COVID activity is high or very high in many states, with levels in the South and West nearing what they were in January (link is external) . As students and teachers head back to school over the next few weeks, Pekosz expects cases will continue to spread before the fall vaccine formulation is rolled out. While infection-induced immunity comes with higher risks than vaccine-induced immunity, this late summer wave will offer good population-level protection that will allow for case rates to trend back down. “I expect we’ll see a little bit of a lull starting around the middle of September or so, before we start seeing the late winter surge of COVID cases,” Pekosz says. Aliza Rosen is a digital content strategist in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health. RELATED: What to Know About COVID FLiRT Variants Understanding the CDC’s Updated COVID Isolation Guidance More Americans Could Benefit from Paxlovid for COVID Infection
Markdown	[Johns Hopkins UniversityEst. 1876 America’s First Research University](https://150.jhu.edu/) [Skip to main content](https://publichealth.jhu.edu/2024/why-covid-cases-rise-every-summer#main-content) - [Expand](https://publichealth.jhu.edu/2024/why-covid-cases-rise-every-summer) [Departments](https://publichealth.jhu.edu/departments) - [Biochemistry and Molecular Biology](https://publichealth.jhu.edu/departments/biochemistry-and-molecular-biology) - [Biostatistics](https://publichealth.jhu.edu/departments/biostatistics) - [Environmental Health and Engineering](https://publichealth.jhu.edu/departments/environmental-health-and-engineering) - [Epidemiology](https://publichealth.jhu.edu/departments/epidemiology) - [Health Policy and Management](https://publichealth.jhu.edu/departments/health-policy-and-management) - [Health, Behavior and Society](https://publichealth.jhu.edu/departments/health-behavior-and-society) - [International Health](https://publichealth.jhu.edu/departments/international-health) - 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Published August 07, 2024 By Aliza Rosen [COVID-19](https://publichealth.jhu.edu/topics/covid-19-and-public-health) [Infectious Diseases](https://publichealth.jhu.edu/topics/infectious-diseases-and-public-health) [Vaccines](https://publichealth.jhu.edu/topics/vaccines-and-public-health) ![An illustration of a wave in the ocean covered in COVID molecules with a person swimming underneath it holding a vaccine](https://publichealth.jhu.edu/sites/default/files/styles/article_feature/public/2024-08/summer-covid-wave.jpg?h=6e972868&itok=hReWAg8L) Fall and winter are known as the time when respiratory viruses surge. When COVID emerged in 2020, it joined flu and RSV as one of the common respiratory viruses that peaks during the colder months. Since then, COVID has transitioned from pandemic to endemic, but has also maintained dual seasonality, peaking twice a year. Every summer since 2020, COVID rates have risen in July and August, due to a confluence of virological, behavioral, and environmental factors. While it’s still too early to say whether this dual seasonality will peter out or become the norm, understanding why these summer surges happen can help us better protect against infection, severe disease, and ruined vacations. Why do we see waves of COVID infections in the summer? Several factors drive summer COVID waves. By mid-to-late summer, many people’s immunity—either from their last vaccination in the fall or from a previous infection—has waned considerably. That, combined with the emergence of [more transmissible variants](https://publichealth.jhu.edu/2024/what-to-know-about-covid-flirt-variants), makes the chance of infection more likely. Human behavior also plays a major role. As the weather gets hot, we spend more time in air-conditioned indoor spaces, where most virus transmission occurs, explains [Andy Pekosz](https://publichealth.jhu.edu/faculty/1972/andrew-stanley-pekosz), PhD, a professor in [Molecular Microbiology and Immunology](https://publichealth.jhu.edu/departments/molecular-microbiology-and-immunology). With the windows closed to keep the cool air inside, we restrict ventilation and air circulation that has shown to reduce virus spread. Many people travel more during the summer, with roughly half of Americans surveyed saying they planned to [travel more, farther, and for longer in 2024](https://www.prnewswire.com/news-releases/tripadvisor-predicts-busy-summer-travel-season-for-americans-an-increasing-desire-for-adventure-302144040.html "(opens in a new window)") than the year prior. Travelers are not only exposed to more people—including from regions with more COVID cases—but also may be more likely to write off mild symptoms as simply the result of jet lag, not illness. Tiredness, headache, or a sore throat—all common after a long flight—are also symptoms of COVID, and without testing to be sure, a traveler may unknowingly expose others. Pekosz experienced this himself after an international trip in July. At first he chalked up feeling tired to the massive time change, but when he started sneezing, he tested at home and it was positive. “I could have easily blown that off and just attributed everything to jet lag,” he says. “With a mildly symptomatic case, I may not have registered that it could be COVID and gone about my business for another couple of days while I was infectious.” Why are there summer waves of COVID infections but not other respiratory illnesses like flu and RSV? COVID’s ongoing dual seasonality can be attributed to both the higher presence of the virus year-round and its ability to mutate. “The number of COVID cases that are present year-round is so much higher than we see for flu or RSV,” Pekosz explains. With that higher baseline, when conditions make for easier transmission—like increased travel and heat driving people indoors—there are more cases to fuel a larger wave of infections. “With flu or RSV, there are so few baseline cases during the summer that these changes in behavior don’t cause the same spread.” Even when COVID rates are relatively low, more cases circulating year-round means more opportunities for the virus to mutate, which SARS-CoV-2 has proven to do very easily. When this happens, new variants emerge that are often more transmissible and better able to evade immunity. “The virus has shown itself to be very malleable, very able to handle mutations,” Pekosz says. How might summer COVID waves impact our vaccination strategies? The 2023-24 respiratory virus season marked a shift in COVID vaccination strategy in the U.S. to one of [annual fall vaccination](https://publichealth.jhu.edu/2023/what-to-know-about-the-updated-covid-19-vaccine-for-fall/winter-2023). That means that—just as with annual flu vaccines—governing bodies like the FDA and CDC use global surveillance data to predict which strains will be circulating and recommend a corresponding vaccine formulation. This happens in the spring so that manufacturers can prepare a vaccine for distribution in early fall. But when infection rates increase, so do the chances for the virus to mutate. A wave of infections in the summer could result in new dominant variants that the vaccine may not protect against as effectively. We saw this evolution play out in real time earlier this year. The FDA’s vaccine advisory panel was scheduled to meet in mid-May to make their recommendations for the fall, but they postponed the meeting to early June to allow more time to monitor the [FLiRT variants](https://publichealth.jhu.edu/2024/what-to-know-about-covid-flirt-variants) that had recently become dominant in the U.S. On June 6, the FDA recommended monovalent [JN.1](https://publichealth.jhu.edu/2024/jn1-the-dominant-variant-in-the-covid-surge)\-lineage vaccines, and then a week later [updated their recommendations to a preference for the KP.2 strain](https://www.fda.gov/vaccines-blood-biologics/updated-covid-19-vaccines-use-united-states-beginning-fall-2024 "(opens in a new window)"), a member of the JN.1 family of variants. “From a public health perspective, it would be great if we could send the same ‘go get your vaccines before respiratory season’ message at one time of year, with the updated vaccines timed to be available then,” Pekosz says. But consistent summer COVID waves could impact those recommendations. “It may end up that some individuals, particularly high risk individuals, are going to be urged to get vaccinated in early summer ahead of a presumed summer wave,” he says. However, it’s unlikely that the upcoming season’s updated vaccines would be available by then, and those individuals would only have access to the previous season’s formulation. The previous season’s formulation would still provide some protection against severe disease and hospitalization, but it likely would have reduced effectiveness against newer variants. Will COVID ever become seasonal like flu and RSV? While the U.S. has experienced a wave of COVID cases every summer since 2020, it’s too early to say whether this is a long-term trend. Many public health experts still expect the virus’s annual spread will eventually look more like those of flu and RSV, which tend to peak during the winter. But exactly how long it will take to settle into that seasonal pattern is not clear. “Historically, there are examples of viruses that do settle into a seasonal pattern after a transition period,” Pekosz says, pointing to the 1918 influenza pandemic, which he says took five or six years to take on the seasonal pattern we’ve come to expect. “We've got to hope that it'll settle into a more seasonal pattern, but we should also start thinking about strategies to deal with it if it doesn't.” Can we expect COVID cases to drop before they rise again in the winter? According to the [CDC’s wastewater surveillance](https://www.cdc.gov/nwss/rv/COVID19-currentlevels.html "(opens in a new window)"), COVID activity is high or very high in many states, with levels in the South and West [nearing what they were in January](https://www.cdc.gov/nwss/rv/COVID19-nationaltrend.html "(opens in a new window)"). As students and teachers head back to school over the next few weeks, Pekosz expects cases will continue to spread before the fall vaccine formulation is rolled out. While infection-induced immunity comes with higher risks than vaccine-induced immunity, this late summer wave will offer good population-level protection that will allow for case rates to trend back down. “I expect we’ll see a little bit of a lull starting around the middle of September or so, before we start seeing the late winter surge of COVID cases,” Pekosz says. Aliza Rosen is a digital content strategist in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health. ## RELATED: - [What to Know About COVID FLiRT Variants](https://publichealth.jhu.edu/2024/what-to-know-about-covid-flirt-variants) - [Understanding the CDC’s Updated COVID Isolation Guidance](https://publichealth.jhu.edu/2024/virologist-explains-the-cdcs-new-covid-isolation-guidance) - [More Americans Could Benefit from Paxlovid for COVID Infection](https://publichealth.jhu.edu/2024/why-more-people-should-be-prescribed-paxlovid-for-covid) ## More from the Bloomberg School - [See latest headlines](https://publichealth.jhu.edu/headlines) - [Learn more about the Department of Molecular Microbiology and Immunology](https://publichealth.jhu.edu/departments/molecular-microbiology-and-immunology) ## Related Content [![Photo of girl smiling at school](https://publichealth.jhu.edu/sites/default/files/styles/related_content/public/2026-04/AdobeStock_310920057.jpeg?h=c6e9cc1f&itok=4lswcnyf)](https://publichealth.jhu.edu/311/2026/school-vaccination-checks-to-improve-coverage) ### [Closing Immunity Gaps Through School Vaccination Checks](https://publichealth.jhu.edu/311/2026/school-vaccination-checks-to-improve-coverage) April 16, 2026 [![A patient fills out a form while her blood and saliva are screened for Hepatitis C and HIV in Spencer, West Virginia, on September 28, 2017.](https://publichealth.jhu.edu/sites/default/files/styles/related_content/public/2026-04/hep-c.jpg?h=4f4378cb&itok=zR0m9KTH)](https://publichealth.jhu.edu/2026/what-to-know-about-hepatitis-c) ### [What to Know About Hepatitis C](https://publichealth.jhu.edu/2026/what-to-know-about-hepatitis-c) April 10, 2026 ### [Why Review of the U.S.’s Measles Elimination Status Has Been Delayed](https://publichealth.jhu.edu/2026/why-review-of-the-uss-measles-elimination-status-has-been-delayed) April 09, 2026 ### [Media Briefing: mRNA Vaccines](https://publichealth.jhu.edu/2026/media-briefing-mrna-vaccines) March 26, 2026 [![Veterinarian seated in front of client with small dog on her lap lets dog sniff his hand while he speaks to the client.](https://publichealth.jhu.edu/sites/default/files/styles/related_content/public/2026-02/pet-vaccines-veterinarian.jpg?h=f321a05b&itok=pykOpVbz)](https://publichealth.jhu.edu/2026/vaccinating-your-dog-or-cat-can-also-protect-you) ### [Why We Vaccinate Our Dogs and Cats](https://publichealth.jhu.edu/2026/vaccinating-your-dog-or-cat-can-also-protect-you) February 27, 2026 Johns Hopkins Bloomberg School of Public Health [615 N. 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Readable Markdown	Fall and winter are known as the time when respiratory viruses surge. When COVID emerged in 2020, it joined flu and RSV as one of the common respiratory viruses that peaks during the colder months. Since then, COVID has transitioned from pandemic to endemic, but has also maintained dual seasonality, peaking twice a year. Every summer since 2020, COVID rates have risen in July and August, due to a confluence of virological, behavioral, and environmental factors. While it’s still too early to say whether this dual seasonality will peter out or become the norm, understanding why these summer surges happen can help us better protect against infection, severe disease, and ruined vacations. Why do we see waves of COVID infections in the summer? Several factors drive summer COVID waves. By mid-to-late summer, many people’s immunity—either from their last vaccination in the fall or from a previous infection—has waned considerably. That, combined with the emergence of [more transmissible variants](https://publichealth.jhu.edu/2024/what-to-know-about-covid-flirt-variants), makes the chance of infection more likely. Human behavior also plays a major role. As the weather gets hot, we spend more time in air-conditioned indoor spaces, where most virus transmission occurs, explains [Andy Pekosz](https://publichealth.jhu.edu/faculty/1972/andrew-stanley-pekosz), PhD, a professor in [Molecular Microbiology and Immunology](https://publichealth.jhu.edu/departments/molecular-microbiology-and-immunology). With the windows closed to keep the cool air inside, we restrict ventilation and air circulation that has shown to reduce virus spread. Many people travel more during the summer, with roughly half of Americans surveyed saying they planned to [travel more, farther, and for longer in 2024](https://www.prnewswire.com/news-releases/tripadvisor-predicts-busy-summer-travel-season-for-americans-an-increasing-desire-for-adventure-302144040.html "(opens in a new window)") than the year prior. Travelers are not only exposed to more people—including from regions with more COVID cases—but also may be more likely to write off mild symptoms as simply the result of jet lag, not illness. Tiredness, headache, or a sore throat—all common after a long flight—are also symptoms of COVID, and without testing to be sure, a traveler may unknowingly expose others. Pekosz experienced this himself after an international trip in July. At first he chalked up feeling tired to the massive time change, but when he started sneezing, he tested at home and it was positive. “I could have easily blown that off and just attributed everything to jet lag,” he says. “With a mildly symptomatic case, I may not have registered that it could be COVID and gone about my business for another couple of days while I was infectious.” Why are there summer waves of COVID infections but not other respiratory illnesses like flu and RSV? COVID’s ongoing dual seasonality can be attributed to both the higher presence of the virus year-round and its ability to mutate. “The number of COVID cases that are present year-round is so much higher than we see for flu or RSV,” Pekosz explains. With that higher baseline, when conditions make for easier transmission—like increased travel and heat driving people indoors—there are more cases to fuel a larger wave of infections. “With flu or RSV, there are so few baseline cases during the summer that these changes in behavior don’t cause the same spread.” Even when COVID rates are relatively low, more cases circulating year-round means more opportunities for the virus to mutate, which SARS-CoV-2 has proven to do very easily. When this happens, new variants emerge that are often more transmissible and better able to evade immunity. “The virus has shown itself to be very malleable, very able to handle mutations,” Pekosz says. How might summer COVID waves impact our vaccination strategies? The 2023-24 respiratory virus season marked a shift in COVID vaccination strategy in the U.S. to one of [annual fall vaccination](https://publichealth.jhu.edu/2023/what-to-know-about-the-updated-covid-19-vaccine-for-fall/winter-2023). That means that—just as with annual flu vaccines—governing bodies like the FDA and CDC use global surveillance data to predict which strains will be circulating and recommend a corresponding vaccine formulation. This happens in the spring so that manufacturers can prepare a vaccine for distribution in early fall. But when infection rates increase, so do the chances for the virus to mutate. A wave of infections in the summer could result in new dominant variants that the vaccine may not protect against as effectively. We saw this evolution play out in real time earlier this year. The FDA’s vaccine advisory panel was scheduled to meet in mid-May to make their recommendations for the fall, but they postponed the meeting to early June to allow more time to monitor the [FLiRT variants](https://publichealth.jhu.edu/2024/what-to-know-about-covid-flirt-variants) that had recently become dominant in the U.S. On June 6, the FDA recommended monovalent [JN.1](https://publichealth.jhu.edu/2024/jn1-the-dominant-variant-in-the-covid-surge)\-lineage vaccines, and then a week later [updated their recommendations to a preference for the KP.2 strain](https://www.fda.gov/vaccines-blood-biologics/updated-covid-19-vaccines-use-united-states-beginning-fall-2024 "(opens in a new window)"), a member of the JN.1 family of variants. “From a public health perspective, it would be great if we could send the same ‘go get your vaccines before respiratory season’ message at one time of year, with the updated vaccines timed to be available then,” Pekosz says. But consistent summer COVID waves could impact those recommendations. “It may end up that some individuals, particularly high risk individuals, are going to be urged to get vaccinated in early summer ahead of a presumed summer wave,” he says. However, it’s unlikely that the upcoming season’s updated vaccines would be available by then, and those individuals would only have access to the previous season’s formulation. The previous season’s formulation would still provide some protection against severe disease and hospitalization, but it likely would have reduced effectiveness against newer variants. Will COVID ever become seasonal like flu and RSV? While the U.S. has experienced a wave of COVID cases every summer since 2020, it’s too early to say whether this is a long-term trend. Many public health experts still expect the virus’s annual spread will eventually look more like those of flu and RSV, which tend to peak during the winter. But exactly how long it will take to settle into that seasonal pattern is not clear. “Historically, there are examples of viruses that do settle into a seasonal pattern after a transition period,” Pekosz says, pointing to the 1918 influenza pandemic, which he says took five or six years to take on the seasonal pattern we’ve come to expect. “We've got to hope that it'll settle into a more seasonal pattern, but we should also start thinking about strategies to deal with it if it doesn't.” Can we expect COVID cases to drop before they rise again in the winter? According to the [CDC’s wastewater surveillance](https://www.cdc.gov/nwss/rv/COVID19-currentlevels.html "(opens in a new window)"), COVID activity is high or very high in many states, with levels in the South and West [nearing what they were in January](https://www.cdc.gov/nwss/rv/COVID19-nationaltrend.html "(opens in a new window)"). As students and teachers head back to school over the next few weeks, Pekosz expects cases will continue to spread before the fall vaccine formulation is rolled out. While infection-induced immunity comes with higher risks than vaccine-induced immunity, this late summer wave will offer good population-level protection that will allow for case rates to trend back down. “I expect we’ll see a little bit of a lull starting around the middle of September or so, before we start seeing the late winter surge of COVID cases,” Pekosz says. Aliza Rosen is a digital content strategist in the Office of External Affairs at the Johns Hopkins Bloomberg School of Public Health. ## RELATED: - [What to Know About COVID FLiRT Variants](https://publichealth.jhu.edu/2024/what-to-know-about-covid-flirt-variants) - [Understanding the CDC’s Updated COVID Isolation Guidance](https://publichealth.jhu.edu/2024/virologist-explains-the-cdcs-new-covid-isolation-guidance) - [More Americans Could Benefit from Paxlovid for COVID Infection](https://publichealth.jhu.edu/2024/why-more-people-should-be-prescribed-paxlovid-for-covid)
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