The past few days have brought alarming news about the state of the pandemic in the US. Hospitalizations from Covid-19 reached new highs in Alabama, Arkansas, California, Florida, Nevada, North Carolina, Oklahoma, South Carolina, Tennessee, and Texas, while case totals have been on the rise in recent weeks for more than half the country. But summer starts this weekend, and there’s still good reason to believe that this infection might be seasonal. If that’s the case, then hot and humid weather could attenuate the spread of the disease. Case counts would fall off. Any “second wave” would be delayed.That’s a good thing, right?
Just a few months ago, we talked of “seasonality” with fingers crossed, as if it were the Covid-19 drama’s version of a deus ex machina. Reuters hinted back in February, when the pandemic’s scope was only just becoming clear, that higher temperatures might “contain the virus.” NPR proposed that maybe—just maybe—summer heat would “crush” the outbreak before it spread too far. Donald Trump, of course, had little interest in the cautious part of any case for cautious optimism: “When it gets a little warmer,” he told supporters at a rally in New Hampshire on February 10, the new coronavirus “miraculously goes away.”
That piece of presidential ballyhoo was met with ample scorn and consternation at the time, but the experts’ gripe was mostly that he’d overstated things. Even many scientists agreed, in principle, that seasonality for Covid-19—if it really did apply—would tend to be a good thing in the short term. “I’m happy to hope that it goes down as the weather warms up,” said senior CDC official Nancy Messonnier two days later, “but I think it’s premature to assume that.” Leaving aside the promise of a miracle, it did seem that any spring or summer slowdown could only help to mitigate the damage, overall. At the very least, it might flatten out the curve (remember that?) and help preserve the health care infrastructure.
But now that we’re on the cusp of summer, and that hoped-for seasonality could be ready to kick in, its implications no longer seem so rosy. In the longer view—looking ahead to fall and winter, too, and then to 2021—this pattern of infectivity could make the virus even more destructive than we thought. If sunlight and humidity do indeed slow its spread, they won’t knock it out completely in the next few months, and that means we should expect a rebound down the line. What’s more, epidemiologists suggest this down-and-up won’t cancel out and be a wash: In fact, the exponential bounceback in the winter would likely overshadow any slight deceleration that happened in June, July, and August. That would be very, very bad.
To be clear, whether the new coronavirus is really seasonal remains unknown. We still haven’t gone through a full year of this pandemic, so it’s impossible to compare how infection rates have waxed and waned in a single location. Even the beneficial effects of humidity are somewhat uncertain; as Maryn McKenna pointed out in WIRED last month, many studies of this question have used laboratory observations, and may not apply in the real world. And to complicate matters further, high heat and humidity may drive people to spend more time indoors, where coronavirus seems to transmit more easily in the air. The science here is still, in many ways, a hot mess.
There are inklings, though, from studies of the past and present, to suggest that weather will indeed modulate the spread of the pandemic. For a paper that came out in April, researchers looked at eight years of data from households in Michigan and found that common respiratory coronaviruses were “sharply seasonal.” Also in April, epidemiologist Marc Lipsitch of Harvard’s T.H. Chan School of Public Health and his collaborators published a study in the journal Science that examined historical data from two of those old coronaviruses. They concluded that the R0 for these diseases—which is to say, how many new cases each infected person generates in the population—hit a peak in winter that was about 30 percent above its lowest point in summer.
More recently, Harvard data scientist Mohammad Jalali and his colleagues arrived at roughly the same estimate for seasonal effects, at least in some places including New York City, with Covid-19. (Their manuscript hasn’t yet been formally vetted by other scientists.) Environmental economist Tamma Carleton of the University of Chicago and her teammates used observations from more than 3,000 different spots around the globe to argue that UV light can change the growth rate of confirmed cases by more than seven percentage points, depending in part on distance from the equator. The seasonal effects would be largest in the far northern and southern latitudes, she and her colleagues suggested, where the length of days varies the most between winter and summer. (The authors of that research, which is also still in draft form, did their best to control for factors such as disparities in Covid-19 testing.)
A few months ago, when the outbreak was exploding, it was tempting to think of these seasonality effects in terms of what would happen first: A downward modulation in the summertime. But the very same effects—of 30 percent, let’s say—could just as well be understood the other way, in terms of boosted viral superspreading later on. It might make more sense to focus on the winterized disease, and what David Fisman of the University of Toronto’s Dalla Lana School of Public Health refers to as a “seasonally juiced” coronavirus. When pandemic waves happen to be synced with the weather conditions in which they thrive—such as fall and winter for influenza—they tend to do more damage, Fisman says. This is what may have happened with the Spanish Flu: Some argue that it first emerged in spring of 1918, when it was out of season for the virus, and then came back the following autumn with far worse effects.
Jalali agrees that the upside of a weakened spread in summer is less consequential than the downside of a boosted spread effect in winter. He’s dismayed that policymakers are relaxing restrictions. “People are going to get to this new norm, and as soon as fall and winter come, we get the reverse effect, and the situation gets worse. That’s the issue,” Jalali says.
The fact that influenza would also strike in winter adds to the concern. It’s not much fun to imagine what a rise in flu and Covid-19 cases would look like if they arrived in concert. Hospitals are stretched to the limits to accommodate even one of these outbreaks. And if the two pathogens can co-infect individuals, that would make the coming season much more deadly, still.
In the meantime, our capacity for dealing with the virus may also wane, in certain ways, with summer. Lipsitch notes that in some hospitals, floors that had been converted to ICUs to take on coronavirus patients are being converted back to their original purposes. “This is not to say we shouldn’t give people medical care [for non-pandemic ailments] while we have breathing space in the summer, but we should also be preparing” for Covid-19 cases to swell again in places where it might be ebbing now, he says. Countries that are entering summer months could take the time to build up more capacity, and stockpile even more personal protective equipment for healthcare workers.
It would be wise to take advantage of the summertime to “crush this virus as close to out-of-existence as possible,” says Jeffrey Shaman, an infectious disease forecaster at Columbia’s Mailman School of Public Health. “That way when it comes into the wintertime and it’s more transmissible you’re starting for a lower setpoint.” The US is far from achieving this, he adds: “We’re not crushing it at all.”
The US’s complacency during the early months of 2020 made it ill-equipped to confront the realities of the Covid-19 pandemic. It wasn’t just the president and his talk of the “miraculous” effects of warmer weather: Many others also figured that a best-case scenario would play out, and the virus would be more or less contained. But we should know by now that it’s better to prepare for the worst-case scenario, even if it’s not a certainty. “There’s a window of opportunity in the Northern Hemisphere right now,” Carleton says. Winter is coming, and we’ve been warned.