No One Really Knows Why COVID Spikes in Summer

Since the pandemic’s earliest days, epidemiologists have been waiting for the coronavirus to finally snap out of its pan-season spree. No more spring waves like the first to hit the United States in 2020, no more mid-year surges like the one that turned Hot Vax Summer on its head. Eventually, or so the hope went, SARS-CoV-2 would adhere to the same calendar that many other airway pathogens stick to, at least in temperate parts of the globe: a heavy winter peak, then a summer on sabbatical.

But three and a half years into the outbreak, the coronavirus is still stubbornly refusing to take the warmest months off. Some public-health experts are now worried that, after a relatively quiet stretch, the virus is kick-starting yet another summer wave. In the southern and northeastern United States, concentrations of the coronavirus in wastewater have been slowly ticking up for several weeks, with the Midwest and West now following suit; test-positivity rates, emergency-department diagnoses of COVID-19, and COVID hospitalizations are also on the rise. The absolute numbers are still small, and they may stay that way. But these are the clear and early signs of a brewing mid-year wave, says Caitlin Rivers, an epidemiologist at Johns Hopkins University—which would make this the fourth summer in a row with a distinct coronavirus bump.

Even this far into the pandemic, though, no one can say for certain whether summer waves are a permanent COVID fixture—or if the virus exhibits a predictable seasonal pattern at all. No law of nature dictates that winters must come with respiratory illness, or that summers will not. “We just don’t know very much about what drives the cyclical patterns of respiratory infections,” says Sam Scarpino, an infectious-disease modeler at Northeastern University. Which means there’s still no part of the year when this virus is guaranteed to cut us any slack.

That many pathogens do wax and wane with the seasons is indisputable. In temperate parts of the world, airborne bugs get a boost in winter, only to be stifled in the heat; polio and other feces-borne pathogens, meanwhile, often rise in summer, along with gonorrhea and some other STIs. But noticing these trends is one thing; truly understanding the triggers is another.

Some diseases lend themselves a bit more easily to explanation: Near the equator, waves of mosquito-borne illness, such as Zika and Chikungunya, tend to be tied to the weather-dependent life cycles of the insects that carry them; in temperate parts of the world, rates of Lyme disease track with the summertime activity of ticks. Flu, too, has pretty strong data to back its preference for wintry months. The virus—which is sheathed in a fragile, fatty layer called an envelope and travels airborne via moist drops—spreads best when it’s cool and dry, conditions that may help keep infectious particles intact and spittle aloft.

The coronavirus has enough similarities to flu that most experts expect that it will continue to spread in winter too. Both viruses are housed in a sensitive skin; both prefer to move by aerosol. Both are also relatively speedy evolvers that don’t tend to generate long-lasting immunity against infection—factors conducive to repeat waves that hit populations at a fairly stable clip. For those reasons, Anice Lowen, a virologist at Emory University, anticipates that SARS-CoV-2 will continue to show “a clear wintertime seasonality in temperate regions of the world.” Winter is also a time when our bodies can be more susceptible to respiratory bugs: Cold, dry air can interfere with the movement of mucus that shuttles microbes out of the nose and throat; aridity can also make the cells that line those passageways shrivel and die; certain immune defenses might get a bit sleepier, with vitamin D in shorter supply.

None of that precludes SARS-CoV-2 spread in the heat, even if experts aren’t sure why the virus so easily drives summer waves. Plenty of other microbes manage it: enteroviruses, polio, and more. Even rhinoviruses and adenoviruses, two of the most frequent causes of colds, tend to spread year-round, sometimes showing up in force during the year’s hottest months. (Many scientists presume that has something to do with these viruses’ relatively hardy outer layer, but the reason is undoubtedly more complex than that.) An oft-touted explanation for COVID’s summer waves is that people in certain parts of the country retreat indoors to beat the heat. But that argument alone “is weak,” Lowen told me. In industrialized nations, people spend more than 90 percent of their time indoors.

That said, an accumulation of many small influences can together create a seasonal tipping point. Summer is a particularly popular time for travel, often to big gatherings. Many months out from winter and its numerous infections and vaccinations, population immunity might also be at a relative low at this time of year, Rivers said. Plus, for all its similarities to the flu, SARS-CoV-2 is its own beast: It has so far affected people more chronically and more severely, and has generated population-sweeping variants at a far faster pace. Those dynamics can all affect when waves manifest.

And although certain bodily defenses do dip in the cold, data don’t support the idea that immunity is unilaterally stronger in the summer. Micaela Martinez, the director of environmental health at WE ACT for Environmental Justice, in New York, told me the situation is far more complicated than that. For years, she and other researchers have been gathering evidence that suggests that our bodies have distinctly seasonal immunological profiles—with some defensive molecules spiking in the summer and another set in winter. The consequences of those shifts aren’t yet apparent. But some of them could help explain when the coronavirus spreads. By the same token, winter is not a time of disease-ridden doom. Xaquin Castro Dopico, an immunologist at the Karolinska Institute, in Sweden, has found that immune systems in the Northern Hemisphere might be more inflammation-prone in the winter—which, yes, could make certain bouts of illness more severe but could also improve responses to certain vaccinations.

All of those explanations could apply to COVID’s summer swings—or perhaps none does. “Everybody always wants to have a very simple seasonal answer,” Martinez told me. But one may simply not exist. Even the reasons for the seasonality of polio, a staunch summertime disease prior to its elimination in the U.S., have been “an open question” for many decades, Martinez told me.

Rivers is hopeful that the coronavirus’s permanent patterns may already be starting to peek through: a wintry heyday, and a smaller maybe-summer hump. “We’re in year four, and we’re seeing the same thing year over year,” she told me. But some experts worry that discussions of COVID-19 seasonality are premature. SARS-CoV-2 is still so fresh to the human population that its patterns could be far from their final form. At an extreme, the patterns researchers observed during the first few years of the pandemic may not prelude the future much at all, because they encapsulate so much change: the initial lack and rapid acquisition of immunity, the virus’s evolution, the ebb and flow of masks, and more. Amid that mishmash of countervailing influences, says Brandon Ogbunu, an infectious-disease modeler at Yale, “you’re going to get some counterintuitive dynamics” that won’t necessarily last long term.

With so much of the world now infected, vaccinated, or both, and COVID mitigations almost entirely gone, the global situation is less in flux now. The virus itself, although still clearly changing at a blistering pace, has not pulled off an Omicron-caliber jump in evolution for more than a year and a half. But no one can yet promise predictability. The cadence of vaccination isn’t yet settled; Scarpino, of Northeastern University, also isn’t ready to dismiss the idea of a viral evolution surprise. Maybe summer waves, to the extent that they’re happening, are a sign that SARS-CoV-2 will remain a microbe for all seasons. Or maybe they’re part of the pandemic’s death rattle—noise in a system that hasn’t yet quieted down.