The Pandemic’s “Perfect and Terrible Storm”

Seasonality and SARS-CoV-2

A family celebrates Thanksgiving as coronavirus circulates unseen in the air around them.
Indoor gatherings increase the risk of SARS-CoV-2 transmission.Art by Niko Yaitanes/Harvard Magazine; images by iStock

Many viruses are known to be seasonal, but as COVID-19 cases in late October reached a record high in the United States, even epidemiologists who have been warning for months that the pandemic will worsen with the approach of Northern hemisphere winter were surprised by the sudden upturn in cases—and the accelerating pace of deaths.

The seasonality of the disease may have lulled many people into believing that their efforts at masking and social distancing were responsible for decreased cases during the summer, according to assistant professor of epidemiology Michael Mina, who is a leading advocate of the use of rapid testing as a public-health measure to suppress outbreaks and allow re-opening of schools and the economy. A better way to think about it, he said during an October 23 call with reporters, is that “We had an opportunity to use and leverage the decreased transmission of the virus during the summer months to prepare for the fall.” Now that the opportunity to deploy vast numbers of cheap tests before a fall reopening has been squandered, “We are really left with impossible decisions,” he said: “either remaining locked down” and dealing with the political and economic fallout; or staying open and reckoning with the health consequences—meaning “some people won’t live as a result.” 

On October 25, Mina tweeted that the 14-day average rate of growth in COVID-19 deaths had leapt from 7 percent to 15 percent in just 3 days, meaning that U.S. deaths were accelerating at a rapid pace. 

Teasing out the “magnitude and mechanisms” of seasonality

As the reopening of society collided with seasonal factors, Mina and other epidemiologists had expected an increase in coronavirus infections and deaths. He had tweeted in early October that “If we do not get this virus under control now, we are in for a perfect and terrible storm.” But even experts have difficulty predicting the extent of seasonality’s impact in the months ahead, because “We don’t know either the magnitude or the mechanisms,” says professor of epidemiology Marc Lipsitch, director of Harvard’s Center for Communicable Disease Dynamics.

Last spring, he and other epidemiologists estimated about a 20 percent difference in transmissibility between the seasonal trough in summer and the peak in winter, based on studies of other coronaviruses. That modest-seeming effect could nevertheless have a significant impact if the use of masks and social distancing in summer, plus a drop in viral activity, ensured that infected individuals transmitted the virus to just one other person on average. Conversely, an increase of 20 percent from that rate would drive an expanding number of infections. 

There are plenty of theories, and enough circumstantial evidence, to suggest that seasonality is driven by multiple factors, including environmental impacts on the virus and the host immune system, as well as human behavior. When temperatures drop below 65 degrees, for example, many people begin to spend more time indoors, where distancing is more difficult, and the lack of ventilation allows airborne viral particles to accumulate. 

Edward Nardell, professor of environmental health and immunology and of medicine, illustrated the latter point during a June 26 talk. Much of the transmission of SARS-CoV-2 is thought be via large respiratory droplets, he said:  “Normally we think of them as settling down within a meter or so of the source and for good measure, we’ve insisted on a six-foot distance between people.” While facemasks capture large respiratory droplets, and even small ones to some degree, evaporation from very small exhaled particles can allow droplet nuclei to become suspended and move with air patterns. Nardell documented the concentration of carbon dioxide (CO2) in an office with five occupants during a three-hour timespan. (Because people exhale CO2, the gas is a good surrogate for the fraction of rebreathed air in a room, and thus the risk of infection from airborne viral particles.) During the first two hours, with the window open, the level of CO2remained fairly constant. But within just 60 minutes of closing the window, the rebreathed fraction of air, and thus the risk of infection, doubled.

Several other factors could play a role in seasonal transmission:

  • Sunlight may modulate the human immune response, perhaps through vitamin D production, regulation of melatonin, or some other mechanism.
  • Temperature may also be important: lab studies of SARS CoV-2 have shown that the virus survives longer in cooler temperatures.
  • Drier winter air could also play a role, perhaps by attenuating the effectiveness of first-line human mucosal defenses, or by increasing the likelihood that small respiratory droplets evaporate to become nearly weightless airborne particles. 

Thanksgiving al fresco?

On the other hand, Lipsitch said to reporters last week, scientists may already know the most important, actionable factors relevant to transmission of SARS-CoV-2: airflow, crowding, mask-wearing, and distance. It may not matter whether someone is in “a grocery store, a fabric store, or a bar,” he said. What may matter most is the number of people, the space between them, the ventilation, and the amount of vocalization.

As the holidays approach, students travel home, and temperatures drive people indoors, Lipsitch is often asked what he would recommend with respect to annual family gatherings. One neighbor asked him in particular what to do about Thanksgiving. His own gathering of 16 family members, he told her, would not be taking place this year. But “people have a very real need to see their families and will have to figure out” how to do that safely. One suggestion he made was to celebrate early, outdoors, with a couple of people at a time: a backyard Thanksgiving in October, or later in the year farther south.

“The guiding principles are all the same,” he continued: “ventilation [by being] outdoors; small groups; staying at a distance; and masking. I don’t think that big holiday gatherings make a lot of sense. So, I would recommend doing as much socializing as you can outdoors while the weather permits.”

Asked why it was important to keep gatherings small, Lipsitch explained that the risk to each person in a group goes up in proportion to the size of the gathering. If there’s a half percent chance that each person is infectious, then in a group of five, there’s approximately a two-and-a-half percent chance that at least one person is infectious. The probability rises as the numbers increase, and at the same time, the chance that the number of people exposed goes up. Roughly speaking, doubling the size of a group increases the risk of transmission by a factor of four—and tripling (from two people to six, for example) raises risk ninefold, “because you have three times as many potentially infectious people and three times as many recipients.” Furthermore, Lipsitch added, “Risk always rises with the prevalence of infection in the places people are coming from.” 

All roads lead to testing

Knowing all this can help individuals, families, and local communities mitigate their risk, but “What we don’t have right now is a comprehensive answer for how to get from high levels of community transmission back down to very low levels without lockdown or intense social-distancing interventions,” Lipsitch continued. “That is the task that lies ahead for [epidemiologists] and I think testing would be at the center of that, but on a much, much larger scale than we have.” Already, he pointed out, “places that have the resources” (including some universities, companies, and the NBA) have successfully controlled transmission with “very extensive testing” together with isolation of infected individuals. The question, he said, is whether the amount of testing that the United States can realistically create overlaps with the amount that would be effective at scale.

Mina, at the forefront of efforts to develop faster, cheaper testing, has met with officials at U.S. regulatory agencies, and says that several significant bureaucratic obstacles to rapid testing have been removed. Still missing, however, is a coordinated federal response, including a commitment to spend the $20 billion that he estimates would be needed to deploy a national testing strategy by January—small change compared to the $16 trillion in estimated economic damages attributable to the pandemic, including the loss of 2.5 million years of potential life so far in the United States alone. “We’re all hoping that a vaccine will create herd immunity,” he continued, but there won’t be enough vaccine for everyone until “late spring or early summer.”

At this point for Mina, it is déjà vu, as once-avoidable increases in hospitalizations threaten to overwhelm the capacity of health-care systems, just as they did in April. “I don’t want to see everything close down again,” he tweeted recently. “I don’t want to see people spending their last days alone.” 

For a discussion of the use of mobile phone data to help control the spread of the virus, listen to the Harvard Magazine podcast, “Ask a Harvard Professor,” with epidemiologist Caroline Buckee.

Read more articles by Jonathan Shaw

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