Julie Bosman and Sarah Mervosh

After weeks of coronavirus patients flooding emergency rooms in Michigan, the worst COVID-19 hot spot in the nation, hospitalizations are finally falling.

On some recent days, entire states, including Wisconsin and West Virginia, have reported zero new coronavirus deaths — a brief but promising respite from the onslaught of the past year.

And in New York and Chicago, officials encouraged by the recent progress have confidently vowed to fully reopen in the coming weeks, conjuring images of a vibrant summer of concerts, sporting events and packed restaurants revving cities back to life.

Americans have entered a new, hopeful phase of the pandemic. Buoyed by a sense that the coronavirus is waning, in part because of vaccinations, more people are shrugging off masks, venturing into restaurants and returning to their pre-pandemic routines. Mayors, governors and other local officials — once the bearers of grim news about the virus’s toll and strict rules for businesses — have joined in the newfound optimism, rapidly loosening restrictions.

Public health experts remain cautious, but said that while they still expect significant local and regional surges in the coming weeks, they do not think they will be as widespread or reach past peaks.

“We’re clearly turning the corner,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

Across the country, the outlook for the pandemic has indeed improved, putting the United States in its best position against the virus yet. The nation is recording about 49,000 new cases a day, the lowest number since early October, and hospitalizations have plateaued at around 40,000, a similar level as the early fall.

Nationwide, deaths are hovering around 700 a day, down from a peak of more than 3,000 in January.

In the past, lulls in the pandemic were short-lived, giving way to the surge across the Sun Belt last summer, and the painful outbreak that stretched across the United States this winter.

But now, there is one crucial difference: More than half of American adults — 148 million people — have received at least one dose of a coronavirus vaccine, perhaps the biggest reason experts are optimistic that the improved outlook may last. Cases, hospitalizations and deaths have also fallen at a time when the weather is getting warmer, which, in many places, will allow people to spend more time outdoors, where the virus spreads less easily.

The situation in the United States stands in stark contrast to other parts of the world, where many countries are still scrambling to secure access to vaccines. India remains in dire crisis, and thousands of people are dying each day in Brazil.

In the United States, even as a sense of hope spreads, there remain strong reasons for caution. The pace of vaccinations is slowing, and experts now believe that herd immunity in the United States may not be attainable. More transmissible variants of the virus are also spreading, threatening to undermine the progress from vaccinations.

That could leave the coronavirus infecting tens of thousands of Americans and killing hundreds more each day for some time. A modeling study released by the Centers for Disease Control and Prevention on Wednesday, citing relaxed restrictions and a new, contagious variant, suggested that cases could tick upward again in the coming weeks, before a sharp drop-off by July. Dr. Rochelle Walensky, the director of the CDC, said, “We are not out of the woods yet, but we could be very close.”

Osterholm pointed to recent outbreaks in Minnesota, Michigan and Oregon as clues to how the pandemic might progress in the coming months. In pockets across the country, small outbreaks have continued to cause alarm. Infections are rising in places like Multnomah County, Oregon, which contains Portland; Pueblo County, Colorado; Grand County, Utah; and Powell County, Kentucky.

“What we’re going to see are more of these localized outbreaks that are going to require a response from governors and mayors,” he said.

It is also possible that the virus could surge again more widely in the fall and winter, when viruses like the flu are typically dominant.

For the moment, though, public health researchers are uncharacteristically optimistic. “We’re in a really good spell and we can act accordingly,” said Andrew Noymer, a public health researcher at the University of California, Irvine, who said it made sense to loosen restrictions now, when the risk is lower than it might be this winter.

Wonderland Camp, a sleep-away camp in Rocky Mount, Missouri, was closed last year, disappointing the children and adults with disabilities who come for arts and crafts, talent shows and a formal dance, complete with a disco ball. But with many staff members and campers vaccinated, and rapid coronavirus tests on hand, the camp is getting ready to open for the summer.

“There is a lot of excitement,” said Jill Wilke, the camp’s executive director, who said this year’s theme, emblazoned on tie-dye T-shirts, would be “Together Again.”

The hopeful outlook has left some cities grappling with new tensions over an old topic: rules around masks. In Kenosha, Wisconsin, this week, the Common Council rejected a push to remove a mask mandate even as the county health department said mass vaccination clinics would soon close because of dwindling demand.

“It’s very tricky,” said Rocco LaMacchia, an alderman who was in favor of ending the mask requirement, saying he believed it should be up to individual business owners. “If I’m walking down the street, not wearing a mask, I don’t want people giving me dirty looks. I think in this community, eventually we’ll all get on the same page, but it’s going to take a lot of doing.”

In parts of New York City, mask wearing has been ubiquitous throughout the pandemic. But even there, the scene is shifting amid CDC recommendations that fully vaccinated Americans no longer need to wear a mask outdoors when alone or in small groups; on recent sunny days, large crowds have flocked to Central Park, and more and more, people are going maskless along sidewalks.

New infections in New York fell by two-thirds in the past month, dipping to around 1,200 new cases a day. Citywide, the number of people hospitalized with the virus recently dropped to below 100.

With 40% of adult New Yorkers fully vaccinated, the city is barreling toward a full reopening. Starting May 19, restaurants, stores, theaters and museums will be allowed to return to near full capacity for the first time since the pandemic began, and tickets for fall Broadway shows will go on sale this week.

Dr. John Swartzberg, an infectious-disease specialist and clinical professor emeritus at the University of California, Berkeley’s School of Public Health, said he was optimistic in the short term, with vaccinations continuing and warm weather luring Americans outdoors.

But he said that he was worried about the virus’s current path of devastation through India and Latin America, and that he wondered if the United States was opening up too quickly, with 50,000 new cases still reported each day. (One year ago, the daily cases were half that number.)

“There is a randomness to the way this virus has spread,” he said. “It flares in one place. It doesn’t progress smoothly through the entire country and entire world. The randomness is what makes me feel insecure.”

In one example of that, Washington state has seen increasing case numbers and hospitalizations in recent weeks, despite rising vaccination numbers and restrictions that have left restaurants and other businesses operating at 50% capacity in much of the state.

Dr. Jeffrey Duchin, the health officer in Seattle and King County, said there was no playbook for an endgame to this pandemic. But he urged people to get vaccinated. “I’m sure all of us want to avoid a long game of whack-a-mole with imposing and easing restrictions,” Duchin said. “Vaccination is the cure.”

States where vaccinations are falling behind — particularly in the South — could be especially prone to outbreaks in the weeks ahead, health experts say. Texas, which was at the center of a harsh outbreak last summer, is trailing the national average in vaccinations, with 39% of people receiving at least one shot. In Mississippi and Louisiana, about a third of people have gotten their first shot.

“Last summer, things were going pretty well around this time,” said Dr. Tara C. Smith, a professor of epidemiology at Kent State University who studies infectious diseases. “As it got a lot hotter in the South and the Southwest, people were inside with air-conditioning, and you saw cases go up. Those are places that are lagging behind a bit in vaccinations. I don’t think it will be as bad as before, but I don’t think this is over yet.”

Still, after a deluge of illness and death over the past year, the headway is encouraging.

Los Angeles County made headlines with the news that it had reported zero new deaths on Sunday and Monday.

The milestone was brief — the county reported 18 deaths on Tuesday — yet it was notable for a metropolis the size of Los Angeles, the nation’s largest county, home to 10 million people. Only a few months ago, hospitals, ambulance services and funeral homes were overwhelmed, and more than 200 people were dying every day in Los Angeles County.

“It’s like day and night,” said Paul Huon, chief executive of Community Hospital of Huntington Park, a hospital there that was so overrun with coronavirus cases this winter that it set up two tents in a parking lot as overflow. Multiple people were dying of COVID-19 during every nursing shift.

Now, the tents are no longer needed, and the hospital is down to two coronavirus patients. Both are expected to survive.