CHICAGO — In recent days, coronavirus cases have been dropping steadily across the United States, with hospitalizations falling in concert. But health officials are growing increasingly concerned that quickly circulating variants of the virus could cause new surges of cases faster than the country is managing to distribute Covid-19 vaccines.

Public health experts likened the situation to a race between vaccination and the virus’s new variants — and the winner will determine whether the United States is approaching a turning point in its battle against the coronavirus, now entering a second year.

“We’re definitely on a downward slope, but I’m worried that the new variants will throw us a curveball in late February or March,” said Caitlin M. Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

Nationwide, new coronavirus cases have fallen 21 percent in the last two weeks, according to a New York Times database, and some experts have suggested this could mark the start of a shifting course after nearly four months of ever-worsening case totals.

This week, the University of Washington’s Institute for Health Metrics and Evaluation, which puts out a predictive model that is widely used for planning, including by some government agencies, released a projection saying new cases in the United States would decline steadily from now on.

“We’ve been saying since summer that we thought we’d see a peak in January and I think that, at the national level, we’re around the peak,” said Dr. Christopher J.L. Murray, director of the institute. Still, Dr. Murray cautioned that variants of the virus could “totally change the story.”

Health officials warned that they have little foresight into what the rest of the winter and spring will bring. President Biden’s new administration has vowed to impose speed and order to what has been a slow, bumpy rollout of vaccinations, in which some 15 million people have received at least one dose of a Covid-19 vaccine. But it is not clear how many vaccines will be available in cities across the country in the coming weeks. The public should still wear masks, officials say, avoid large gatherings and sign up to be vaccinated as soon as they are eligible.

Some experts, looking abroad at how new viral variants sent cases surging in Britain, Ireland, South Africa and northern Brazil, said the United States could merely be in a lull before a new spike begins. Even after an epidemic’s peak, it remains dangerous: Sometimes just as many people are infected after the peak as were before.

“I think the next three months could be the worst part of the pandemic,” said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “I hope I’m dead wrong.”

Nicolas A. Menzies, one of several scientists running the Prevention Policy Modeling Lab at Harvard’s T.H. Chan School of Public Health, which tracks levels of herd immunity, said he felt it was “more probable than not” that infections would climb again.

It is important to spot regions where variant strains are turning up, he said, since they would be the most likely to have early surges. Thus far, the British variant and the new “California variant” have been found most often in Southern California and Florida, according to the Centers for Disease Control and Prevention. Cases are slowly declining in both regions. But it’s “still too early to tell,” he said.

As the Biden administration on Thursday announced a “full-scale wartime effort” to combat the virus, Dr. Anthony S. Fauci, the nation’s top infectious-disease expert, said the nation’s outbreak “looks like it might actually be plateauing in the sense of turning around,” but he cautioned that the country remained in a dire situation.

Thirty-seven states are seeing sustained reductions in cases, with only one reporting significant increases. Arizona and California, which reached disastrous new case records in recent weeks, have reported noticeable drops over the past several days. Around some Midwestern cities that drove surges of infections in the early fall, case numbers have fallen 50 percent or more from their peaks.

Still, the country continues to average nearly 190,000 new cases each day, more than any point of the pandemic before December. Deaths from the coronavirus are still extraordinarily high, with more than 4,300 deaths announced on Wednesday, the second-highest daily total of the pandemic. And in some places, there has been no progress at all.

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Answers to Your Vaccine Questions

While the exact order of vaccine recipients may vary by state, most will likely put medical workers and residents of long-term care facilities first. If you want to understand how this decision is getting made, this article will help.

Life will return to normal only when society as a whole gains enough protection against the coronavirus. Once countries authorize a vaccine, they’ll only be able to vaccinate a few percent of their citizens at most in the first couple months. The unvaccinated majority will still remain vulnerable to getting infected. A growing number of coronavirus vaccines are showing robust protection against becoming sick. But it’s also possible for people to spread the virus without even knowing they’re infected because they experience only mild symptoms or none at all. Scientists don’t yet know if the vaccines also block the transmission of the coronavirus. So for the time being, even vaccinated people will need to wear masks, avoid indoor crowds, and so on. Once enough people get vaccinated, it will become very difficult for the coronavirus to find vulnerable people to infect. Depending on how quickly we as a society achieve that goal, life might start approaching something like normal by the fall 2021.

Yes, but not forever. The two vaccines that will potentially get authorized this month clearly protect people from getting sick with Covid-19. But the clinical trials that delivered these results were not designed to determine whether vaccinated people could still spread the coronavirus without developing symptoms. That remains a possibility. We know that people who are naturally infected by the coronavirus can spread it while they’re not experiencing any cough or other symptoms. Researchers will be intensely studying this question as the vaccines roll out. In the meantime, even vaccinated people will need to think of themselves as possible spreaders.

The Pfizer and BioNTech vaccine is delivered as a shot in the arm, like other typical vaccines. The injection won’t be any different from ones you’ve gotten before. Tens of thousands of people have already received the vaccines, and none of them have reported any serious health problems. But some of them have felt short-lived discomfort, including aches and flu-like symptoms that typically last a day. It’s possible that people may need to plan to take a day off work or school after the second shot. While these experiences aren’t pleasant, they are a good sign: they are the result of your own immune system encountering the vaccine and mounting a potent response that will provide long-lasting immunity.

No. The vaccines from Moderna and Pfizer use a genetic molecule to prime the immune system. That molecule, known as mRNA, is eventually destroyed by the body. The mRNA is packaged in an oily bubble that can fuse to a cell, allowing the molecule to slip in. The cell uses the mRNA to make proteins from the coronavirus, which can stimulate the immune system. At any moment, each of our cells may contain hundreds of thousands of mRNA molecules, which they produce in order to make proteins of their own. Once those proteins are made, our cells then shred the mRNA with special enzymes. The mRNA molecules our cells make can only survive a matter of minutes. The mRNA in vaccines is engineered to withstand the cell’s enzymes a bit longer, so that the cells can make extra virus proteins and prompt a stronger immune response. But the mRNA can only last for a few days at most before they are destroyed.

Virginia is reporting some of its highest infection numbers yet. New outbreaks are raging in South Carolina. And in parts of Texas, including around San Antonio and along portions of the Mexican border, case numbers are as high as they have ever been. The county that includes Laredo is reporting more than 500 cases each day, a per capita figure more than twice as high as Los Angeles County, which is also struggling.

In places that have seen a slowing of new cases in recent days, local and state health officials were sharing positive — but tentative — news about the virus.

“Everything’s moving the right way,” a smiling Dr. Allison Arwady, the commissioner of public health for Chicago, said at a news conference on Thursday, noting that because of encouraging metrics in the city, museums have reopened, gyms are allowing group classes and more restrictions could be loosened in the coming days. Epidemiologists say that cases rise and fall in cycles controlled almost entirely by human behavior, and some experts worried that new openings of businesses, permitted because of sinking case numbers, might just set off new surges once more.

Gretchen Musicant, the Minneapolis commissioner of health, said that officials in the state were “encouraged, but wary” of the situation, and that they continue to be watchful as Minnesota begins reopening certain sectors of the economy once again.

“We’re watching to make sure that those reopenings don’t escalate our rates again,” Ms. Musicant said.

As epidemiologists warn about the spread of new variants, health officials are racing to vaccinate as many people as possible. As of Thursday, nearly 2.4 million people had been fully vaccinated. More than half of states had administered less than 50 percent of the doses shipped to them.

Dr. Megan Ranney, an emergency physician in Rhode Island, said that since vaccinations began rolling out to health care workers last month she had seen a dip in the number of her colleagues getting sick. “That, to me, is the first real glimmer of hope,” she said.

But because there are so many factors at play in the spread of the coronavirus, including the behavior of the public and their willingness to stay at home, “it doesn’t feel durable,” Dr. Ranney said.

There was optimism among health experts that deaths from the virus, which in the United States have reached levels in January higher than at any other point in the pandemic, may soon slow some.

A new study found that coronavirus patients in hospitals run by the Department of Veterans Affairs were twice as likely to die if they were treated in intensive care units stretched to capacity as those treated during quieter times.

Public health experts had hoped that first vaccinating the groups at highest risk of death or most likely to be exposed to the virus would result in fewer deaths among those infected. But if new virus variants lead to significantly more infections, “it’s going to result, eventually, in more deaths,” said Andrew Noymer, an associate professor of public health at the University of California, Irvine.

North Dakota, which once had the country’s worst rates of coronavirus infection, has seen its cases slow in recent weeks. In November, 116 Covid-19 patients were being treated across three floors of a Sanford Health hospital in Fargo. On Thursday, there were 10 coronavirus patients in the hospital.

“It has dropped dramatically,” said Melanie Allen, 43, a registered nurse and clinical care leader in the hospital’s coronavirus unit.

The crush of Covid-19 patients may have eased, giving Ms. Allen a few days off to decompress and begin home-renovation efforts such as painting her bathroom. But she is still on edge, knowing how quickly cases can spike if the declining numbers once again lull people into a false sense of security.

“I still get a little nervous when I see people going out and doing things, restrictions being lifted,” she said. “I don’t want people to start letting their guard down.”

Julie Bosman reported from Chicago, and Donald G. McNeil Jr. from New York. Reporting was contributed by Giulia McDonnell Nieto del Rio and Amy Harmon from New York, Mitch Smith from Chicago and Jack Healy from Denver.

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