Seasonality
In temperate climates, colds and influenza tend to wax in the fall and winter and wane in the spring and summer. Why exactly this is remains a topic of much research and debate, but it stands to reason that Covid-19 — caused by a coronavirus, as are many colds — would settle into a similar pattern. To some extent it already has: this fall and winter have been the worst of the pandemic in North America and Europe. But as of now, there isn’t enough evidence to make confident pronouncements about what the coming of spring will bring.
Immunity
In the U.S., close to 30% of the population has probably already been infected with the SARS-CoV-2 virus. Another 12% (there must be some overlap with the first group, but probably not too much yet) has already received at least a first dose of vaccine. About 40% of the population thus has at least some protection. At the current pace of vaccination, this share is increasing by another percentage point every three days, and with the weekly U.S. vaccine supply about to rise from 11 million doses to 13.5 million, it will be increasing faster than that soon.
The threat of new variants, combined with the inevitable imperfections in the immunity provided by both previous infections and vaccines, mean that getting to the full herd immunity that keeps Covid-19 from spreading even if we’re all hanging out together maskless indoors is a daunting and perhaps impossible task. But 40% of the population possessing some immunity should still slow the spread of the disease, and getting to 50% and then 60% should slow it even more.
There’s also the matter of who has been vaccinated. Residents of nursing homes and other long-term care facilities have accounted for an estimated 36% of U.S. Covid-19 deaths. As of mid-January, almost 80% had received at least a first vaccine dose. Americans 65 and older have accounted for 81% of deaths, and they’re the main focus of the current inoculation effort. The vaccines approved for use in the U.S., as well as a couple of others likely to be approved soon, appear to be 100% effective in preventing deaths from Covid-19. So even if they can’t stop a resurgence of Covid this spring, they may be able to stop it from being nearly as deadly as past waves.
Putting It Together
At the moment one can find quite a wide range of views among experts about what is likely to happen next. Michael Osterholm of the University of Minnesota’s Center for Infectious Disease Research and Policy sees B.1.1.7 causing a surge in the U.S. akin to a “category 5 or higher” hurricane. Michael Mina of Harvard’s T.H. Chan School of Public Health thinks that “the next few months could start to offer a reprieve. Ideally that will last through the summer until we get into next fall, when we’ll probably have another wave of it.” In Germany, Christian Drosten of the Charité medical university’s Institute of Virology contemplates a scenario in which, once the elderly have been vaccinated,
there will be immense economic, social, political and perhaps also legal pressure to end the corona measures. And then, huge numbers of people will become infected within just a short amount of time, more than we can even imagine at the moment. ... It will, of course, be primarily younger people who are less likely than older people to have severe symptoms, but when a huge number of younger people get infected, then the intensive-care units will fill up anyway and a lot of people will die.
Israel, which will be ending its latest lockdown this weekend, looks to be the first test of Drosten’s theory. Thanks to the world’s most successful vaccination campaign, virtually everyone older than 60 has already received two shots. But Israel still has many more Covid cases on a per-capita basis than the U.S., and there are now more people younger than 60 hospitalized with Covid there than over 60. The next few weeks could be really interesting.
In the U.S., the “ensemble” of the various forecasts collected for the Centers for Disease Control and Prevention by researchers in biostatistician Nicholas Reich’s laboratory at the University of Massachusetts at Amherst foresees continued declines in cases and in deaths. The ensemble forecast only goes out four weeks, though, and the Reich lab’s handy visualization tool makes clear that it hasn’t been great at calling turning points in the past.
The University of Washington’s Institute for Health Metrics and Evaluation has a forecasting model that looks four months into the future, which got it a lot of attention early in the pandemic but also a lot of criticism because it 1) was repeatedly wrong and 2) eschewed epidemiological modeling for a simple exercise in curve-fitting. Since then, IHME has incorporated an SEIR model and seemingly become much better at forecasting, with its dire September warning of 410,000 total Covid-19 deaths in the U.S. by Jan. 1 not far off from the eventual CDC count of 382,051. At the moment, IHME is projecting a sharp increase in infections in March and April — albeit not quite back to late-December/early-January levels even in the worst-case scenario — and a much-less-severe resurgence in deaths.
