COVID-19's summer wave is back — and it's becoming as predictable as flu season
Just as many Americans are squeezing in the last few weeks of summer, COVID-19 is once again making its presence known. Health experts say the virus has settled into a familiar rhythm: a rise in cases each summer and winter, and this year's warm-weather uptick — often called the “summer surge” — is already underway.
“COVID-19 is rising nationally but from a low baseline. As of the latest CDC data, infections were growing or likely growing in 47 states, led generally by the South and West,” Dr. Syra Madad, an infectious disease epidemiologist at Harvard's Belfer Center and chief biopreparedness officer for New York City's public hospitals, told USA TODAY. “The increase became apparent in parts of the South and West during July and more clearly nationwide by late July and early August.”
Wastewater surveillance paints a similar picture. Data from WastewaterSCAN, a Stanford-led monitoring network that tests sewage samples across the country, shows COVID-19 levels in the “high” category and rising. August concentrations were up 105% compared with July, and levels have roughly doubled each month since June.
“Since we began monitoring COVID-19 in wastewater, we have seen a consistent pattern: virus concentrations peak in the summer and winter months,” said Amanda Bidwell, scientific program manager for WastewaterSCAN. “We're still seeing lower SARS-CoV-2 concentrations than at this time last year, but sites are trending upward nationwide. In the West and South, we've already surpassed the concentrations observed during the winter peak.”
COVID-19 is classified as a respiratory illness, much like influenza or RSV, but it no longer behaves like them. Instead of a single yearly “season,” the virus has carved out a distinct two-peak pattern — one in the summer, one in the winter — that has become increasingly predictable over time.
When COVID-19 first emerged in 2020, it was a novel pathogen. Human immune systems had no prior exposure or memory of it, making the entire population vulnerable. Over time, however, our bodies have built up layers of immunity through infection and vaccination, and the virus's transmission patterns have settled into something more cyclical — resembling the seasonal trends seen with other infectious diseases.
Data from COVID-NET, the CDC's hospitalization tracking system, shows a consistent uptick in cases beginning in July and stretching into September, a pattern visible as far back as the 2022 season.
“If you actually look at it, it starts year after year at a national level. It's been practically identical,” said Dr. Michael Mina, chief medical officer at biopharmaceutical company Invivyd and a former assistant professor at Harvard T.H. Chan School of Public Health. “Sometime in early July, or mid-July, you start to see an upswing, and then it really picks up speed.”
Unlike the flu and RSV, which tend to spike once a year in the colder months, COVID-19 now surges twice annually. While the pattern is well established, the underlying drivers remain a subject of debate among experts.
Part of the explanation is likely a combination of environmental and behavioral factors, said Mina. “Humans have similar types of contact rates. Just the way that humans bump into each other across the population is pretty consistent from year to year,” he said. “We can find it's transmitting because of the way that humans aggregate indoors during the winter, for example.”
During the peak of summer, that means hotter regions push people into air-conditioned indoor spaces with windows closed and limited ventilation. In late summer, it means more mixing among families and school-aged children, who carry viruses back and forth between classrooms and homes. By fall, Mina says, there's often a lull.
“That's because we're kind of mitigating some of these transmission opportunities by maybe opening up windows more in the fall and congregating less inside, or having less air conditioning — things along those lines,” he noted.
Dr. Andrew Handel, a pediatric infectious diseases specialist at Stony Brook Children's Hospital, told USA TODAY that the virus's evolution is also fueling the summer uptick. New variants keep emerging, and immunity from prior infections or vaccinations wanes over time, creating windows of vulnerability.
“The virus tends to mutate quickly, but there are small mutations one at a time, and so those can kind of add up over time more than we see with other viruses like influenza,” Handel said. “With COVID, we also have a shorter-lasting immunity … There's enough of a waning in immunity that six months after your initial infection in the winter, people tend to get another maybe less robust, less severe infection in the summer.”
While “summer surge” is a catchy phrase, both Handel and Madad caution that the July-to-August increase is more accurately called a “summer wave” or “summer rise.” The true surge, in terms of severe illness, still happens in the winter months.
“Emergency-department visits remain very low, so this is best described as an early, relatively small summer wave, not a major surge,” said Madad. “Particularly when infections are increasing but severe illness remains low.”
Still, she emphasized, “Low national severity does not mean no risk. Older adults, infants and immunocompromised people remain particularly vulnerable.”
Looking ahead, Mina believes the summer wave could eventually become a less significant phenomenon over the next few decades as immunity and vaccination science improve. Babies born to mothers with existing antibodies, and children exposed to the virus at a young age, tend to develop more robust immune responses than adults encountering a new pathogen for the first time, he explained.
As more children gain exposure to the virus and as immunization coverage expands, population-level immunity should continue to strengthen, reducing the frequency of breakthrough infections over time.
In the meantime, Mina advises staying up to date on recommended immunizations, staying home when sick, and taking sensible precautions to reduce transmission.
Madad added that anyone feeling ill should take a COVID test, and consider additional measures such as improving indoor air ventilation and filtration, practicing good hand hygiene, asking a doctor about antivirals if infected, and wearing a well-fitted mask after exposure, during local outbreaks, or when around vulnerable people.
“The message is proportionate vigilance, not alarm,” she said. “COVID is rising but we have effective tools to reduce transmission and severe disease.”