Coronavirus cases in the U.S. have been plummeting since their recent peak in mid-September, and practically everyone has grown tired of COVID precautions. But cases plateaued in early November, and winter is coming. Experts warn it is not safe to let down our guard just yet.

Some western states, such as Alaska, Colorado and North Dakota, have become COVID hotspots as colder weather moves activities indoors, suggesting another national surge may be ahead. And the U.K., often a harbinger of COVID trends in the U.S., recently saw its biggest jump in new cases since the waning of the Delta surge there.

So, what should the U.S. expect this winter? Although it is impossible to predict COVID’s future with any certainty, one thing is clear: the country’s second COVID winter will be different from its first. “Prepare for somewhat less of a disaster,” says Vaughn Cooper, an evolutionary biologist at the University of Pittsburgh who researches the evolution of infectious diseases. But even less of a disaster could still be pretty bad.

COVID surged to its highest levels last winter, killing nearly 250,000 people in the U.S. during the months of December, January and February. This winter the country should fare better, thanks to vaccines that are highly effective against severe illness and death. But more than 40 percent of the U.S. population is not fully vaccinated, and more than 30 percent have not received any doses. “That’s more than enough to sustain a raging pandemic, with a really transmissible virus,” Cooper says. The currently circulating Delta variant is more than twice as contagious as the version of SARS-CoV-2 that was predominantly circulating this time last year.

Averting a Surge Is ‘In Our Hands’

Ali Mokdad, an epidemiologist at the University of Washington’s Institute for Health Metrics and Evaluation (IHME), a group that developed a prominent model of COVID trends, says he expects to see U.S. cases start to rise in late November, resulting in another wave. But the size of that wave depends on a variety of factors.

For example, the IHME team modeled a worst-case scenario showing a massive surge if 100 percent of vaccinated people stop wearing masks, if people start to move around more regardless of vaccination status, and if variants spread twice as fast as Delta. In a best-case scenario, the wave disappears entirely—and an estimated 56,000 lives are saved—but only if 95 percent of Americans wear masks. Somewhere in the middle lies the likeliest scenario, in which cases decline but see a post-holiday bump with a projected peak of 1,200 daily deaths in mid-February before falling again.

“There is nothing written in stone here,” Mokdad says. “We can change it, and it’s in our hands.”

Indeed, what is happening in the U.K. offers insight into how circumstances within and outside of one’s control—such as waning immunity, seasonality, changes in human behavior, and new variants—might cause cases to spike in the U.S.

“I feel fairly gloomy about the winter,” says Sarah Rowland-Jones, an immunologist at the University of Oxford and an infectious disease physician. “The rates of COVID have been steadily increasing in the U.K. since the release of restrictions in July, and we haven’t even started with flu season yet.” She has noticed one improvement over last winter, however: far fewer COVID patients are ending up in the intensive care unit, and those that do tend to be unvaccinated.

Spending More Time Indoors

Now, fears are mounting that the protection provided by COVID vaccinations may be starting to fade. Rich countries including the U.K. and the U.S. were among the first to roll out vaccines, which means they could also be the first to see vaccine effectiveness wane. But Rowland-Jones, whose lab investigates immunity to viruses, is not convinced waning immunity can explain the spike in cases seen in the U.K. “There is lots of research looking at the dynamics of antibody and T cell [immune] responses after vaccination, and those levels are maintained surprisingly well after vaccination, so there isn’t a massive drop-off there,” she says.

Still, there is enough of a decline that Rowland-Jones thinks giving booster shots—at least to particularly vulnerable populations—before winter is a good idea; she just thinks getting first shots into more of the general population is a bigger priority. The U.K. has lagged behind the U.S. in vaccinating adolescents, and now the pandemic there appears to be driven by cases among 10- to 19-year-olds. One of the more favorable forecasts for the U.S., produced by the COVID-19 Scenario Modeling Hub, assumes about half of children of ages five to 11—for whom the Pfizer vaccine was recently authorized—will soon be vaccinated. The model suggests vaccinating children will help reduce overall COVID cases, shaving off more than 400,000 infections from a potential surge.

Another reason some experts have concerns about this impending winter is the virus’s seasonality. Respiratory viruses such as colds and the flu tend to spread more easily in cold, dry air; data suggest COVID might behave the same way. But many scientists believe that at the current stage of the pandemic, shifts in human behavior during the winter season will play a bigger role in increasing transmission than the weather itself. As temperatures drop, people spend more time indoors, often in close contact with others. “You have to remember this is an indoor talking disease,” Cooper says.

Crowded indoor gatherings and the lifting of nearly all coronavirus restrictions could be contributing to the worrisome levels of COVID in the U.K. Conditions are also changing in the U.S., albeit more slowly. Schools are back to in-person lessons, and many districts are dropping mask mandates. A recent Kaiser Family Foundation poll found that a little less than half of the U.S. public (43 percent) have resumed their normal prepandemic activities. Of the more than 1,500 U.S. adults surveyed, few plan to skip large gatherings (22 percent) or avoid travel (15 percent) this holiday season.

Globally, mobility is back to levels seen before the COVID-19 pandemic began, according to cell phone data the IHME uses to inform its forecasts. Mokdad does not find that encouraging. “This virus is not going away. And when we look at history, every time cases started coming down, Americans were more likely to move around and less likely to wear masks,” Mokdad says.

The Threat of New Variants

Yet Mokdad and other experts do not think a massive surge like the one forecasted in IHME’s worst-case scenario is likely, as long as a new variant does not emerge that makes COVID even more transmissible.

That is what happened last winter in the U.K. when the emergence of the Alpha variant drove a surge in infections. SARS-CoV-2 has since spawned other variants—such as Beta, Gamma and Mu—but its run through the Greek alphabet appears to have stalled at Delta, the highly contagious variant that is now responsible for more than 99 percent of COVID cases worldwide. 

Cooper says that because of Delta’s dominance, any new variants of concern will likely be daughters of Delta: genetic descendants that contain additional and potentially more dangerous mutations. One such variant—which many are dubbing Delta plus—has already gained ground in the U.K, where it accounts for an estimated 6 percent of cases. Delta plus appears to be slightly more transmissible than the original Delta, but it is too early to say how much the former contributed to the recent rise of cases in the U.K.  

Evolution is still difficult for SARS-CoV-2, says Cooper. Many studies have shown that even though mutations arise within infections, most of them do not become widespread. But when there are a lot of unvaccinated people and a large number of infections, he says, “it’s effectively like an evolutionary windstorm over many smoldering embers of little hopeful mutants that might normally go extinct.”

Whether any of these embers will fuel a larger fire is anybody’s guess. But Cooper and others think that this winter, both the worst-case and best-case scenarios could play out at the same time in different parts of the country. The pandemic will likely flare up in regions of the U.S. with lower vaccination rates and less mask wearing and diminish in places that are better protected.

“I think we’re all in agreement that there is no will for another major shutdown, so we’re going to roll with it,” Cooper says. “And the question is, are we all, as a country, as a society, rolling with it in a sensible and prepared way?”