All across the US, people are hugging, talking in each other’s faces, going to the office, attending indoor sports events and not wearing masks in the Walmart. Yet the SARS-CoV-2 virus that causes Covid-19 continues its retreat, with confirmed cases down by more than 50% over the past month.
Most of the credit for this wonderful turn of events has — correctly — gone to vaccines, with more than 60% of US adults now having received at least one dose, 94% of them the BioNTech-Pfizer and Moderna mRNA vaccines that have proved to be even more effective than their spectacular clinical-trial results indicated last fall.
But there is another factor behind the case decline that’s getting less attention than it probably should. That is, it’s late spring in the Northern Hemisphere, with the sun shining and temperatures warming even in parts of the country where spring tends to be especially tardy. As with influenza and the coronaviruses that cause common colds, there appears to be a seasonal element to the spread of the SARS-CoV-2. Which means that, as the days shorten and temperatures cool in a few months, there’s a good chance that case numbers will start rising again.
I’m not predicting a repeat of last winter. Thanks to those vaccines, and the surely substantial percentage of unvaccinated Americans who have already had Covid-19, the nation’s immune systems will be at least partially armed against whatever comes. The same should be true in other temperate regions such as Europe.
But I do worry that, by not talking more about the disease’s seasonality, public health officials risk stumbling into a situation in which late-summer and early-fall school, office and entertainment re-openings are followed by a rise in cases that makes it seem like everything is spiraling out of control again. Far better to signal now that we may have to retrench somewhat come fall. Or as former Food and Drug Administration Commissioner Scott Gottlieb, who as a private citizen has offered some of the smartest and most consistent public-health messaging throughout the pandemic, put it a few weeks ago: “I don’t think we’re going to be having holiday parties in, you know, the back room of a crowded restaurant on December 20.”
Why don’t experts and officials talk more about Covid-19’s seasonality? I have two theories. One is that, as with so many aspects of the US Covid response, it’s an overreaction to something stupid that Donald Trump said. The other is that experts don’t like talking about Covid seasonality because they can’t really explain it.
The stupid thing that Trump said was:
You know, a lot of people think that goes away in April with the heat — as the heat comes in. Typically, that will go away in April.
That, from Feb. 10 of last year, was the first in a long series of presidential assertions that Covid was about to disappear, which in turn spurred infectious-disease experts to declare again and again that no, it wasn’t. Those experts were of course right on the whole, but the pandemic did fade in April in New York City and other hard-hit places, and mostly stayed away there until October, although it was impossible to tell how much of the decline was due to seasonality and how much to behavior change and acquired immunity. Meanwhile, the summer coronavirus wave in the US was concentrated in places where (1) not many had been infected in the spring and (2) it’s so hot in summer that people spend more time indoors then than they do in the spring or even winter.
A highly infectious disease to which few have immunity can spread even when seasonal conditions are against it. That happened with the H1N1 influenza pandemic in the summer of 2009 — incidence did decline in July and early August in the US, but remained well above normal summer levels of influenza activity before taking off again — and SARS-CoV-2 appears to have been much more contagious than H1N1 even before the various mutations that have since increased its transmissibility.
As for people spending lots of time indoors, that’s seen as a major reason why colds and influenza spread faster in winter. Crowded and poorly ventilated offices, restaurants and entertainment venues provide a congenial environment for viruses. So do schools, although young children seem to play less of a role in spreading Covid-19 than they do colds and the flu.
Increased time indoors is the first major category of explanations for the seasonality of respiratory viruses. The second has to do with humidity: when there’s less of it, viral particles can survive and remain airborne longer. The third relates to people’s immune systems, which are weaker in the winter because they get less Vitamin D from the sun.
That’s my rephrasing of a breakdown offered last year by epidemiologist Marc Lipsitch, director of the Center for Communicable Disease Dynamics at Harvard’s T.H. Chan School of Public Health. A multi-author run-through in the 2020 Annual Review of Virology gave a similar taxonomy of (1) seasonal changes of environment, (2) human behavioral patterns and (3) viral factors, emphasizing that temperature as well as humidity might play a role in virus survival (warmer air can hold more water, so they’re not exactly unrelated).
Another possibility, discussed in the Proceedings of the National Academy of Sciences in January, is that changes in ultraviolet radiation, which has been shown to inactivate SARS-CoV-2 in the lab, might explain some Covid-19 seasonality.
Having so many possible explanations isn’t necessarily a good thing. As Lipsitch and Cecile Viboud of the National Institutes of Health’s Fogarty International Center alliteratively argued in a 2009 discussion of influenza seasonality:
This potpourri of possible mechanisms places us in a kind of Popperian purgatory, in which data in support of every hypothesis exist, yet none of the hypotheses has been subjected to tests that are rigorous enough to reject it.
Karl Popper was the philosopher who argued that to be scientific, a hypothesis had to be falsifiable — there had to be a way to prove it wrong. Lipsitch and Viboud were actually commenting approvingly on a paper by Jeffrey Shaman and Melvin Kohn that offered such a hypothesis on the role of humidity in influenza transmission, one that has since been tested with mostly positive results. It’s less clear that it applies to Covid-19, though. “At this point the evidence is incredibly muddled,” says Shaman, now a professor of environmental health sciences at Columbia University’s Mailman School of Public Health.
The seasonal patterns haven’t held up as one gets closer to the equator, which makes sense given that there aren’t really seasons close to the equator but does raise even more questions about the role of heat and humidity in slowing Covid’s spread. Also, there are some common-cold viruses that are known to spread best in fall, spring or summer, so seasonality can mean different things for different respiratory diseases. In an era in which researchers were able to sequence the genome of SARS-CoV-2 in a few days, and design effective vaccines based on that knowledge, the study of Covid seasonality feels a bit confused and unscientific. Which may be one reason you don’t hear scientists talking about it all that much.
Still, Covid-19 “probably does have some seasonality to it,” allows Shaman. His research group at Columbia has been deeply involved in modeling and forecasting the spread of the disease, and includes a seasonal factor based on the past behavior of the cold-causing coronavirus OC43 in some of its models but not others.
The well-known Covid-19 forecasting model maintained by the University of Washington’s Institute for Health Metrics and Evaluation, which took a lot of deserved flak for its methods and errors early in the pandemic but since September has proved a pretty reliable guide to the disease’s medium-term US trajectory, also incorporates seasonality. It foresees continued declines in US daily Covid infections until mid-July, followed by a doubling up to Sept. 1, the current forecast end date. In an article published in the Journal of the American Medical Association in March, IHME Director Christopher Murray and London School of Hygiene and Tropical Medicine Director Peter Piot warned that “the public and health systems need to plan for the possibility that Covid-19 will persist and become a recurrent seasonal disease.”
Such seasonal recurrence is what scientists generally have in mind when they say we aren’t going to reach the herd-immunity threshold for Covid-19, the point at which immunity is so widespread that each case of the disease leads to less than one additional case, on average. Right now each case is in fact leading to less than one additional case, but if seasonality is playing a significant role in keeping transmission down then we can expect this to reverse in the fall. Whether the disease wave that follows would be akin to a normal cold and flu season, a bad flu season or something worse is something no one knows yet. Risks may also increase in subsequent years as immunity fades and new variants take hold.
The pandemic appears to be ending in the US, and that’s awesome. But the aftermath of pandemics can still be dangerous. The deadliest US flu seasons since 1918-1919, for example, were in 1919-1920 and 1928-1929.