A mid-year wave might be brewing for the fourth year in a row. Will it always be like this?
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.
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