Is the current rise in COVID-19 hospitalizations really the result of people moving indoors to enjoy air conditioning? Experts and journalists often make this assumption when they’re quoted in the media about the virus’s small summer “waves” or “surges” (which, this year, is really more like a small uptick). Similarly, when there’s a winter surge, it’s usually attributed to people flocking inside to escape the cold.

But whether it’s summer or winter, the explanation falls flat. Because regardless of the season, humans are generally an indoor species. And the virus is still evolving new ways to get around our immunity, most recently spinning off a new omicron subvariant called EG.5.

While there’s good evidence that the virus spreads more easily indoors, there’s very little evidence that people are spending much more time indoors now than they were in the spring. Nor is there much evidence that most people ever spend enough time outside to affect the global ebb and flow of SARS-CoV2 or other respiratory viruses.

People live 90% of their lives indoors, said Joseph Allen, director of the healthy buildings program at Harvard School of Public Health. He’d like to see more emphasis on ventilation and filtration of indoor air to protect people from wildfire smoke and pollutants as well as viruses.

On an individual level, it’s true that people are less likely to contract a virus at a park or beach than at a crowded indoor party. But on a societal level, attributing surges in virus to people moving indoors doesn’t make much sense. So what actually causes viral diseases to wash over us in waves or in seasonal peaks?

Michael Osterholm, an epidemiologist at Center for Infectious Disease Research and Policy, says there’s more to learn about the ecology of viruses – how they interact with each other and with our immune systems. He’s been critical of explanations that attribute all surges to changes in our behavior. Even when waves rose in the winter, they often fell long before the weather changed enough to allow more outdoor activities.


With COVID, of course, a series of new variants had fueled a number of past waves, though again, it’s never been clear what causes those waves to subside.

And there is still some mystery behind the seasonal activity of more familiar viruses. Influenza cases follow a seasonal pattern in both the Southern and Northern hemispheres, peaking in winter, but in the tropics, there’s flu all year round. Why? Osterholm said it’s not well understood.

Even weirder is the fact that new viruses can push old ones right out of their seasonal slot. The world saw very little RSV and ordinary influenza during the winter of 2020-2021. Then those infections returned in 2022, but they peaked in the fall instead of the winter.

Osterholm says he wishes doctors and journalists would stop using the term “tripledemic” to describe the co-existence of flu, RSV and COVID in late 2022. COVID-19 was on the way out, and the other diseases weren’t worse than usual but simply peaked early.

While some scientists attributed the lack of flu and RSV cases in 2020 and 2021 to mask-wearing, Osterholm is doubtful. For one thing, attempts at universal masking weren’t effective enough to suppress COVID during the surges.

And the same displacement of other viruses happened during the 2009 H1N1 flu pandemic. “If you look at RSV, it just literally disappeared, and that carried well into 2010, and we saw no other flu viruses. Why? There were no mitigation strategies going on,” he said. Nobody was masking.


Osterholm said he didn’t earn any popularity points in late 2020 and early 2021 for saying the darkest days of the pandemic might be ahead of us. But omicron showed he was right to be concerned about the impact of potential new variants.

Another new variant might emerge any time, and the current variant, omicron, continues to spin off new subvariants, the BA series giving way a series of XBB variants, recently giving way to variants called EG.5 and EG.5.1. Osterholm said he is still studying the impact of these new variants, which are on the rise around the world and became dominant in the U.S. last week. The federal Centers for Disease Control and Prevention is less equipped than it was last year to monitor new variants, because of a tapering-off of efforts to gather genetic sequences of samples.

The rise in cases highlights an uncomfortable reality: We don’t – and have never had – complete control over the pandemic, even though we do have tools to reduce risk for individuals.

Early in the pandemic, there was a popular notion that we’re failing a group project, the goal of which would be to conquer the virus. But ending the pandemic before the vaccines were available wasn’t realistic, and the experts led people in too many directions – not all of them helpful. Many people stayed home, disinfecting mail and groceries while railing against beachgoers and images of Swedish people in outdoor cafes. Much of that effort and the outrage was misplaced and not anchored by anything like scientific evidence.

Now that airborne transmission is well understood, people can reduce the risk to elderly friends and relatives by holding weddings and other big gatherings outdoors. People can also reduce their risk of contracting or transmitting COVID-19 (and a lot of other viruses) by wearing a tight-fitting, high-quality mask, staying home if they feel ill and washing their hands a lot. And of course, vaccines can reduce the risk of an illness becoming serious. Booster shots can be important for those at high risk.

Scientists should keep searching for an evidence-based explanation for the rise and fall of virus waves, large and small. It’s an intriguing question and the answer could prove useful. And having a clear answer would be a lot more satisfying than admitting we just don’t know.

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