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It’s that time of year: a thick, oppressive heat blankets everything, people huddle inside air-conditioned homes, offices, shops and cafes for respite—and COVID is surging again.

Levels of SARS-CoV-2, the virus that causes COVID, have increased in wastewater samples across the U.S., with the biggest uptick in the West. The percentage of positive tests—though not a perfect metric because people aren’t testing as much—has also increased, but hospitalizations have remained relatively low. Most viral respiratory infections, such as influenza, peak in the winter. But for the four years that SARS-CoV-2 has circled the globe, it has caused peaks not just in winter but every summer, too. The question is, why?

Possible reasons for the summer COVID peak are complex, but they fall into three main categories: characteristics of the virus itself, characteristics of its human hosts, and environmental factors.

SARS-CoV-2 continues to evolve new variants. One rises to the fore every six months or so, according to Peter Chin-Hong, a professor of medicine at the University of California, San Francisco, who specializes in infectious diseases. In recent weeks, several new subvariants of the virus’s Omicron variant have emerged as dominant—including the so-called “FLiRT” variants (such as KP.2 and KP.3), as well as a newer variant called LB.1. These variants may be slightly more transmissible or better at evading the immune system than previous ones, Chin-Hong says.

Human behavior and the environment are other likely drivers of summer surges. During the summer, many people gather for events, travel for vacations or simply spend more time inside to beat the heat. The Northern Hemisphere winter has a string of holiday gatherings that are perfect for

spreading disease; likewise, “in the summer, it’s Father’s Day, graduation, Fourth of July, and then summer travel,” Chin-Hong says. “It’s kind of a like a one-two-three punch.”

“We know that nearly all [COVID] transmission happens indoors, in places with poor ventilation and/or poor filtration,” says Joseph Allen, an associate professor at the Harvard T.H. Chan School of Public Health and director of the Harvard Healthy Buildings Program. “One hypothesis is that these building factors and human behavior are driving the summertime increases in cases.” Although many offices or other large buildings have an HVAC system that can pull in fresh air from outside, many houses and apartment buildings with window-mounted air conditioners do not. Instead, these ACs simply recirculate stale, virus-laden air inside a room.

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