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 casesand 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 assistantprofessor of epidemiology Michael Mina, who is a leadingadvocate of the use of rapid testingas 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 thatWe 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 consequencesmeaning some people wont live as a result.

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

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 seasonalitys impact in the months ahead,because We dont know either the magnitude or the mechanisms, says professor of epidemiology Marc Lipsitch, director of Harvards 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 thatinfected individuals transmitted the virus to just one other person on average. Conversely, an increase of 20 percent from that rate would drive anexpandingnumber 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, weve 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:

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 yearfarther south.

The guiding principles are all the same, he continued: ventilation [by being] outdoors; small groups; staying at a distance; and masking. I dont 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 theres a half percent chance that each person is infectious, then in a group of five, theres 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 fourandtripling(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.

Knowing all this can help individuals, families, and local communities mitigate their risk, but What we dont 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 Januarysmall change compared to the$16 trillion in estimated economic damagesattributable to the pandemic, includingthe loss of 2.5 million years of potential lifeso far in the United States alone. Were all hoping that a vaccine will create herd immunity, he continued, but there wont be enough vaccine for everyone until late spring or early summer.

At this point for Mina, it is dj vu, as once-avoidable increases in hospitalizations threaten to overwhelm the capacity of health-care systems, just as they did in April. I dont want to see everything close down again, he tweeted recently. I dont want to see people spending their last days alone.

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Seasonality of the SARS 2 Coronavirus - Harvard Magazine