In fact, the latest data may reflect that. Epidemic curves posted by the US Centers for Disease Control and Prevention that showed a steady upward trend for two months have begun turning down; between September 10 and 16, hospitalizations shrank 4.3 percent (though deaths increased, by 2.7 percent). That downturn can’t have been created by the newest Covid boosters, because they were only released September 13.
But the degree to which people accept the new shots might control whether and when a winter surge arrives. “We know from this virus, year over year, people’s immune response to each vaccine or boost starts waning at that six- to eight-month time point,” says Mark Cameron, an associate professor of population and quantitative health sciences at Case Western University.
Ashish Jha, a physician who is the dean of the Brown School of Public Health and served for 14 months as the White House’s Covid-19 response coordinator, said at a media briefing last week, “My expectation is we’re going see a further decline for probably the next month or two, and then we’re going to see the virus starting to rise again, as we get into the holidays and beyond.”
To say that a virus is seasonal seems self-evident: at a particular point in the year, cases begin; at some further point, they subside. But “seasonality” conceals mysteries, even for the flu. Environmental changes—in ambient temperature, humidity, or the duration of UV light—might combine to create optimal conditions for the flu’s return. So might anatomical responses to those changes, such as the effect of colder or drier air on mucous membranes and the epithelium of the respiratory tract. Equally, so might behavioral shifts: crowding indoors to escape the colder weather, and sharing spaces that offer less air circulation than the summertime outdoors.
If the complex effects of all those influences aren’t well-understood for influenza, one of the most-studied viruses, imagine the knowledge gaps that exist for Covid. They include not just the conditions that influence the flu and winter colds (caused by an array of pathogens including other coronaviruses), but also the evolutionary behavior of SARS-CoV-2 itself. It is still a mystery why the Delta variant emerged when it did, and why the much more divergent Omicron variant took over from it. It is even more mysterious why the Omicron variant has remained so dominant nearly two years later.
“The question is: Why has it settled on that and not made another major seismic move to a brand-new variant?” asks Robert Bednarczyk, an infectious disease epidemiologist and associate professor at Emory University’s Rollins School of Public Health. “If we can understand where that stability is coming from, it will be very helpful to plan moving forward.”
If Covid were stable and seasonal—or at least gained predictable periodicity in arrival and mutation—planners could follow the decades-old model built for the flu. A large, global, durable infrastructure—led by the World Health Organization but assisted by national governments and academic researchers—detects, analyzes, and forecasts the evolution of influenza viruses early enough to formulate vaccines for the following season. That infrastructure can only operate because of the predictability of the flu’s annual return.
