Fall has arrived, flu shots are rolling out in pharmacies, and pediatricians are watching for an uptick in respiratory syncytial virus, or RSV. In other words, it’s virus season. Covid deaths and hospitalizations also began rising at the end of July, and wastewater surveillance that looks for the virus has been on a slow upward trend.

So do we have a “Covid season” now? It’s an important question, because knowing when cases will surge could help public health officials and health care administrators plan for vaccines, treatments, and hospital staffing—and might prompt everyone else to be a little more self-protective.

But experts on the front lines and doing data analysis say it’s too soon to declare that Covid has achieved seasonality. Looking back over the previous three years, they do see patterns: a spike at some point in the summer, such as the arrival of the Delta variant in 2021, and a spike sometime in the late fall or winter, such as the Thanksgiving surge of Omicron later that year. But those spikes haven’t occurred at the exact same time from year to year, and it’s possible they didn’t all arise for the same reasons.

“You might look at that data and think, maybe this is just a biannual virus, compared to flu and RSV, which have single seasonal peaks,” says Cameron Wolfe, an infectious disease physician and professor at the Duke University School of Medicine. “But that gets much harder to say when you factor in that as a society we behave very differently, seasonally. And that we’ve behaved differently in different years of the pandemic, according to how restricted we were in terms of our movements, how much mitigation we were actually performing, and how immune we were, either by vaccine or native infection.”

In other words, what looks like a season might be an artifact created by our behavior, not the virus’s. The way our bodies react to SARS-CoV-2 might also be playing a role in pushing it around the calendar.

“As we get more used to seeing this virus, our immunity builds up a little more and a little more—so the time between the winter surge and the summer surge gets longer and longer,” says David Dowdy, a physician and professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. “What may eventually happen is that it gets longer and longer, until it’s every winter. It may be very interesting this year to see if we have the same winter surge of Covid, because we had such a late summer surge. There’s going to be a fair amount of immunity still in the population.”

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.

A similar infrastructure could be built to prepare for Covid, too. Predicting the virus’s likely arrival could ensure that fresh boosters are developed and shipped well in advance of a surge, and get to where they are needed. Trustworthy predictions of Covid’s future behavior could also exert more subtle effects, allowing drug manufacturers to envision demand and hospitals to stress-proof capacity.

“Paxlovid and other antivirals, monoclonal antibodies, whatever we’re using to treat Covid—we’d want to start ramping up production of those drugs in the late summer, so we have them around in the winter, within their shelf life,” says Jacob Simmering, a health economist and assistant professor at the University of Iowa’s Carver College of Medicine, and coauthor of a March analysis that found reliable seasonal spikes in cases in the United States and Europe. “That should influence production decisions. And it also has implications for the healthcare system: making sure we have resources, staff availability, beds.”