Why COVID Spikes in the Summer, and How to Stay Safe

WebMD

22-07-2025

With cases climbing again, you might be wondering why a "winter virus" keeps returning in the heat, and what you should be doing right now to protect yourself.
"A couple of months ago, experts were uncertain about a summer wave," said Jodie Guest, PhD, an epidemiologist at Emory University in Atlanta who has tracked COVID since the start of the pandemic. "The low levels of respiratory illness in spring 2025 and stable variant landscape suggested a quiet summer."
But three key indicators now have COVID watchdogs sounding the alarm: a new virus strain, increasing wastewater levels, and emergency room visit upticks in parts of the U.S.
The CDC now lists the "COVID-19 epidemic trend" as growing or likely growing for more than half of U.S. states. The strongest indicators were seen in Arkansas, Hawaii, Illinois, Iowa, Kentucky, North Carolina, Ohio, Pennsylvania, Texas, and Virginia.
Here's what to know about summer COVID, and how to protect yourself – especially if you have a summer vacation or travel coming up.
I thought COVID was a big risk during flu season. Did something change? No, COVID has always had two waves, about six months apart – one in the summer and the other at the height of winter "respiratory season," when flu and respiratory syncytial virus (RSV) also rage.
The winter wave typically peaks sometime between "December and February, coinciding with colder weather and increased indoor gatherings," said Guest, senior vice chair of the Department of Epidemiology at Emory's Rollins School of Public Health. "Summer waves have occurred sporadically, often driven by new variants or waning immunity."
Here's when COVID peaked the past three summers:
2022: Around Aug. 1
2023: Around Sept. 30
2024: Around Aug. 31
This timing has prompted some experts to theorize that summer waves may be linked to people spending more time indoors with air conditioning during the height of summer heat.
Why are there two COVID waves, but influenza only spikes once per year? Some scientists point to rapid mutations and waning immunity from past infection or vaccination.
"A big chunk of people will get sick in a wave, and they'll have pretty good immunity to that particular variant, and that immunity is enough to sort of stop that wave in its tracks," said Emily Landon, MD, an infectious disease specialist at UChicago Medicine in Illinois. The virus then needs to change enough to evade existing immunity before another cycle starts back up.
A second theory suggests there are three distinct groups of people – a summer group, a winter group, and a group that gets infected twice a year. The idea is based on the idea that people vary in how long they're immune after infection, vaccination, or both.
Scientists still need more data to know for sure, "but the bottom line is we're certainly seeing a big increase in cases in the summer and a big increase of cases in the winter," Landon said.
Is this surge caused by the new variant? Yes, partly. NB.1.8.1 – or Nimbus – has been on the rise in Asia recently, and it now accounts for as many as 43% of new cases in the U.S., up from 24% at the beginning of June, according to CDC data. It's the variant known for a " razor blade" sore throat symptom, although it doesn't appear to cause more severe illness than other versions of the virus, Landon said.
This geographic pattern has become pretty set: A rising variant in Asia or Europe typically foreshadows a rise in the U.S., said Sabrina A. Assoumou, MD, MPH, an infectious disease doctor and professor at Boston University Chobanian & Avedisian School of Medicine.
Another regional clue: "We have typically seen that the [U.S.] rise in cases starts in the South," said Assoumou. She noted that some of the highest wastewater levels of SARS-CoV-2 (the virus that causes COVID) are in Florida and Alabama right now. "This is often followed by increases in other parts of the nation such as the Northeast."
How can I protect myself from summer COVID?
Make sure your vaccination is current. If you got a COVID shot last winter, you're probably good, Landon said. But if you're 65 or older, immunocompromised, or have a health condition that puts you at high risk – the CDC website maintains a list of qualifying conditions – ask your health care provider if you need a booster now. People with upcoming travel should consider a booster, too. Landon, who has rheumatoid arthritis and receives treatment that affects her immune system, just got a booster because she's going on vacation soon. "I want the best protection since I know that we're seeing the beginning of a summer spike," she said.
Remember that masks are still an effective tool. If you're traveling, wear a mask like an N-95, KN-95, or KF-94, and make it as snug as you can tolerate. Don't just wear it on the plane; wear it while lining up to board, too. It is OK to slip it down to sip a drink or eat a snack while in flight. "I would absolutely recommend that everyone wear a mask on an airplane," Landon said. "The last thing you want is even any kind of cold, let alone COVID, when you're on vacation."
Pack some COVID tests in your travel bag. If you know you have COVID, you can consider starting antiviral medication right away, which can reduce your risk of hospitalization.
Maybe bring a Paxlovid prescription too. If you're older or have a condition that puts you at high risk, ask your doctor to prescribe Paxlovid or another antiviral called molnupiravir to bring with you in case you get sick.
Ask sick people to stay home. If you're planning a summer party, Landon suggested adding a note to the bottom of the invite that says, "We have some high-risk people coming. If you're not feeling well, we'll take a pass and meet up with you another time when you're feeling better." Some people feel obligated to attend parties even when they aren't feeling well. "Letting them know that they're off the hook if they're sick and that you really don't want them to come if they're sick is a really important way that you can help protect your own health and help other people to do the right thing," Landon said.