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Cold weather favors the coronavirus. But as summer gives way to fall, infectious-disease experts are guardedly optimistic that the spread of COVID-19 this autumn and winter won't be as brutal as in the previous two years of the pandemic.

Coronavirus scenarios from multiple research teams, shared in recent weeks with federal officials, foresee stable or declining hospitalizations in early fall. The scenarios show the possibility of a late-fall surge. A new variant remains the biggest wild card. But several factors - including the approval this week of reformulated boosters and the buildup of immunity against the latest strain of the virus - could suppress some of the cold-season spread, experts say.

"There's sort of even odds that we would have some sort of moderate resurgence in the fall. But nothing appears to be projecting anything like an omicron wave," said Justin Lessler, a University of North Carolina epidemiologist who helps lead the collection of covid-19 planning scenarios from a group of research organizations.

The scenarios assume that reformulated vaccine boosters will be embraced by the public at a rate similar to that of the annual flu shots - possibly an optimistic assumption given that more than half of Americans eligible for boosters have yet to receive their first dose.

Peter Marks, the top vaccine official at the Food and Drug Administration, said in a briefing Wednesday that the approval of reformulated boosters comes as the agency is "looking at a possible fall wave, with a peak around December 1st."

Predictions about the pandemic rarely age well. In the United States, the pandemic appeared to be winding down in May 2021 amid a vigorous vaccination campaign, only to get wound up again with the rise of the new variants.

The emergence of a new variant in September could result in a wave of infections and severe illness in December, according to Lauren Ancel Meyers, director of the University of Texas Covid-19 Modeling Consortium. A variant emerging in October would push the peak to January, she said.

Any new variant that could change the pandemic's trajectory would have to be more transmissible than the omicron subvariant BA.5 currently circulating. It might emerge from an obscure branch of the virus's family tree - which is exactly what happened last November, when omicron, with its stunning package of mutations, appeared in southern Africa and immediately overtook the reigning delta strain.

Vaccines remain highly effective at lowering the infection fatality rate and keeping people out of the hospital, and the Biden administration continues to lean heavily on vaccination and boosting as the most powerful weapon against the virus. Anthony Fauci, the president's chief medical adviser for the pandemic, told The Washington Post that the fall campaign against the virus will demand widespread uptake of booster shots.

"We're not going to eradicate it. We're not going to eliminate it," Fauci said. "But we do have the capability to get it to a low enough level so that it doesn't continue to disrupt the social order."

The federal government, meanwhile, is turning much of the fight against the virus over to the private sector. As of Friday, the government would no longer mail free coronavirus tests to the public. The plan is to transition the payment of treatments to insurers, pharmacy benefit managers, hospitals and patients themselves by the middle of next year. Updated boosters have already been purchased by the federal government and will remain free to consumers.

But booster uptake so far has been underwhelming. Of the 62 million people over the age of 50 who are eligible for a second booster, only 22 million have received it so far, according to CDC data. Of the 95 million people between 18 and 49 who are eligible for their first booster, only 38 million have availed themselves of it.

Some may be waiting for the reformulated vaccine before rolling up their sleeves again. But covid apprehensiveness is not what it once was, and many may feel that a couple of shots is enough.

Moreover, some people may need help getting access to an additional shot, said Brown University epidemiologist Jennifer Nuzzo. She would like to see stronger messaging from the government to encourage vaccination.

"The most important thing we can do - top, top, top of my list - is make sure that everyone who is at high risk is up to date with their vaccinations," she said.

The CDC is reporting about 82,000 new covid cases daily, on average, although the true number of infections is assumed to be many times higher because so many people test themselves at home. The more reliable number is hospitalizations, currently about 30,000 patients, according to the CDC. Both numbers are trending downward, as is the death toll - a daily average of 387, according to the CDC. (Average daily deaths peaked above 3,300 in January 2021, as the virus spread in an overwhelmingly unvaccinated population, and topped 2,600 per day the following winter amid the omicron wave.)

If no new coronavirus variant emerges, the numbers should stay stable or decline until the new year, the report from Lessler's forecasting group states.

The most pessimistic scenario is that a new variant will appear and the booster campaign will get rolling late, resulting in a projected 1.3 million hospitalizations and 181,000 deaths over a nine-month period (August 2022 to May 2023), compared with 700,000 hospitalizations and 111,000 deaths in the most optimistic scenario, with no new variant and an early start to the booster campaign.

Dylan George, director of operations at the CDC's recently established Center for Forecasting and Outbreak Analytics, compares disease modeling to weather forecasting. The agency looks at many models, incorporating variables to create a wide range of plausible scenarios. Right now, he said, the CDC believes that the BA.5 subvariant is cresting in most of the country.

Behavior is another variable in the equation. Precautions have largely been relaxed for much of the country. Many companies are requiring workers to report to the office but no longer require vaccination or provide regular coronavirus testing. Schools have dropped mask mandates.

"People are not wearing masks," George said. "People are running around in bigger groups. People are traveling more. Schools are not having any kind of mitigation. Will that impact spread in a bigger way as well?"

Waves of infection are to some degree self-limiting. The virus "burns through all the susceptibles," as George put it, losing momentum. But then time passes, and immunity wanes. Vaccine-based immunity against infection appears to drop significantly in a matter of months, even as protection against severe disease continues.

Another complication is the presence of other circulating viruses, including influenza, which also has a cold-weather seasonal signature.

"There's all sorts of respiratory things, especially as we go into the school season," George said. "How is flu going to play out now that we're all coming together? . . . There has always been concern about the 'twindemic.' "

Fauci noted that, following a cascade of new subvariants earlier this year, the BA.5 omicron subvariant and the almost identical BA.4 have not been challenged this summer by a new strain. Immunity against BA.5 and BA.4 has been steadily building in the population as people get infected and then recover. That immunity should get a significant enhancement from the new boosters that have been designed to fight not only the original strain of the virus, but also BA.5/BA.4.

"I don't think it's going to be a major surge if it stays BA.5," Fauci said of the hypothetical fall wave of cases.

Amid a broader return to normal behavior there remains a significant contingent of people who are covid-cautious - aware that hundreds of people a day are still dying from the virus - and continue to wear masks indoors or limit contacts with others.

Millions are now experiencing the health crisis of "long covid," an array of post-infection symptoms that include severe fatigue and brain fog. It is a slippery disease to diagnose conclusively because many symptoms could signal long covid or a different ailment. One CDC report said 1 in 5 infected people develop long covid.

Marks, the FDA official, said he routinely fields calls from people in their 20s and 30s with long covid symptoms, and said the illness represents a serious public health challenge. "The brain fog, in some cases, the mood changes - people who used to be very bright and cheery, who now are anxious and depressed - those things seem to be very real," he said.

Some patients have struggled with long covid for more than two years and have been unable to return to work or resume their pre-pandemic way of life.

"Some are young, healthy, athletic people, and they can't even go back to work," said Akiko Iwasaki, an immunologist at Yale University School of Medicine. "People should know the risk before they remove the mask and stop getting their boosters."

Evidence points to the virus settling into a seasonal pattern, said Columbia University epidemiologist Jeff Shaman. Viral transmission is enhanced by the low humidity of the indoor environment during the winter as well as by the decline in sunshine and its sterilizing ultraviolet radiation, Shaman said.

He worries that the virus could continue to sicken and kill people at rates higher than the seasonal flu, which, according to CDC data, took between 12,000 and 52,000 lives per year between 2010 and 2020. If the covid mortality continues at the same rate as it has been over the past five months, that would be roughly 120,000 deaths a year, Shaman calculated. If that is the new normal, it's sobering, he said.

Infectious-disease experts don't want to tempt fate with sunny forecasts. The coronavirus is still adapting to people as it mutates randomly, and natural selection favors the most immune-evasive strains.

"My forecast is that you can't really forecast," Fauci said. "It is such an unpredictable virus in the sense that we've been fooled before, and we likely will continue to be fooled."