The U.S. approaches Thanksgiving. As Thanksgiving Approaches, the U.S.

CHICAGO — A month ago, new coronavirus cases in the United States were ticking steadily downward and the worst of a miserable summer surge fueled by the Delta variant appeared to be over. But as Americans travel this week to meet far-flung relatives for Thanksgiving dinner, new virus cases are rising once more, especially in the Upper Midwest and Northeast.

Federal medical teams have been dispatched to Minnesota to help at overwhelmed hospitals. Michigan is enduring its worst case surge yet, with daily caseloads doubling since the start of November. Even New England, where vaccination rates are high, is struggling, with Vermont, Maine and New Hampshire trying to contain major outbreaks.

Nationally, case levels remain well below those seen in early September, when summer infections peaked, and are below those seen last Thanksgiving. But conditions are worsening rapidly, and this will not be the post-pandemic Thanksgiving that Americans had hoped for. More than 90,000 cases are being reported each day, comparable to early August, and more than 30 states are seeing sustained upticks in infections. In the hardest-hit places, hospitalizations are already climbing.

“This thing is no longer just throwing curveballs at us — it’s throwing 210 mile an hour curveballs at us,” said Michael Osterholm, an epidemiologist at the University of Minnesota. He said that the virus had repeatedly defied predictions and continues to do so.

The new rise in cases comes at a complicated moment. Last Thanksgiving, before vaccines were available, federal and local officials had firmly urged Americans to forgo holiday gatherings. But in sharp contrast, public health officials, including Dr. Anthony S. Fauci, the nation’s leading infectious-disease expert, have mostly suggested this year that vaccinated people could gather in relative safety.

In interviews across the country, Americans said they were not sure what to think.

Jess Helle-Morrissey, 43, a therapist who lives in St. Paul, Minn, said she has decided to host a dinner, though case rates in her state are among the country’s worst. About 4,200 cases are emerging every day and hospitalizations are soaring in Minnesota.

“They are diligent maskers and don’t take any extra risks,” she said of her guests. “Everyone who is coming, I keep saying, is, you know, vaxxed to the max.”

In important ways, the country is in better shape than during previous upticks. Doctors have learned more about how to treat the virus and experts are hopeful that antiviral pills will soon be approved. Most crucially, many Americans have been vaccinated. The availability of those shots — including the recent approval of booster doses for all adults — has raised confidence for many who said they planned to proceed with holiday celebrations.

But about 50,000 coronavirus patients are hospitalized nationwide, and tens of millions of Americans have declined to be vaccinated. The course of the virus in Europe, where Austria is entering a lockdown and some areas of Germany have shut down Christmas markets, has raised fears about just how high case numbers might rise in the United States.

“The last thing I want is what Austria is doing,” said Dr. Allison Arwady, the public health commissioner in Chicago, where cases have started to rise. “I really, really don’t want to go there.”

In Austria, about 66 percent of the population has been fully vaccinated against the virus. In the United States, about 59 percent of the population has been.

Still, millions of Americans were forging ahead with holiday plans. Federal officials expected Thanksgiving air travel to approach prepandemic levels. And plenty of people who hit the road this year will be unvaccinated, unmasked and largely unworried about Covid-19.

Many experts said the wide availability of vaccines, now authorized for everyone 5 and older, as well as at-home testing, made it possible for vaccinated people to host a relatively safe, though not fully risk-free, gathering.

Dr. Arwady said she planned to spend the holiday with extended family members, all of whom are vaccinated except young children who are not eligible. While reports of new cases in Illinois have increased 62 percent in the last two weeks, she said she wanted vaccinated people to feel confident going about their life and to enjoy Thanksgiving.

“Is there the potential for some spread? Of course there is,” said Dr. Arwady, who suggested that unvaccinated adults consider staying home. “Are the people who are vaccinated, even if they haven’t gotten a booster, likely to end up in the hospital or die? They’re really not.”

Dr. Osterholm said he worried about breakthrough cases in vaccinated people who did not have booster shots and about the potential for future mutations of the virus. Still, he too said he would gather for the holiday with vaccinated family members who live nearby.

Many others who were interviewed, including in states with some of the highest infection rates, voiced exhaustion and frustration that the virus was even a consideration this holiday season, 20 months into the pandemic.

In New Mexico, which is averaging 1,400 cases a day, Bernice Medina, 37, a food truck operator, said she was uneasy when she gathered with her large family for the holidays last year but felt safer now because she was vaccinated. In Michigan, home to nearly one of every 10 new coronavirus cases nationwide, Dustin Johnston, 40, a photographer, said the vaccines made him confident enough to gather locally with older relatives.

“The vaccination, I think, changes everything,” said Mr. Johnston, whose state has the country’s highest rate of recent cases.

Officials who once urged caution were now deferring to individuals to make their own decisions.

“It’s really hard to tell people to stay away from their families,” said Mayor Katie Rosenberg of Wausau, Wis., where cases have surged to their highest levels since late 2020. “I can’t anymore.”

Dr. Rebecca Smith, an epidemiologist at the University of Illinois, said she planned to travel by vehicle with her children to see family but would get tested before and after.

“People want to get back to normal and we understand that — and there are ways to do that safely,” she said.

Still, Dr. Smith said she expected the outbreak in Illinois to continue to worsen as the virus rips across Midwestern and Northeastern states that largely avoided the worst of the summer surge. In the last two weeks, reports of new cases have increased by more than 40 percent in Pennsylvania, by more than 80 percent in Massachusetts and by 70 percent in Indiana.

Infection levels are also persistently high across much of the West, including in Arizona and New Mexico, where hospitalizations are rising, and in Alaska and Wyoming, which have started to improve after enduring major outbreaks. But case rates in California are relatively low, as they also are in the South, the region hit hardest over the summer.

Ahead of Thanksgiving 2020, the country was reporting 175,000 new infections a day and was midway through its worst case surge of the pandemic. Vaccines were still weeks away from being authorized, many schools were closed and at-home rapid tests were rare. But even as scientists warned that Covid-19 was unlikely to completely vanish, there was widespread optimism back then that vaccines could make the virus an afterthought in daily life.

“It was wicked bad last year during the holidays,” said Kirk Burrows, 26, a paramedic in Unity, Maine, who said he planned to stay home for another Thanksgiving. “I think it’s going to be worse this year.”

Mr. Burrows, who described long ambulance rides with coronavirus patients being transferred to hospitals hours away, said he thought many people had let their guard down as the pandemic persisted. Maine is routinely reporting more than 700 new cases a day, its most since the pandemic started, and hospitalizations have reached record levels.

“I think a lot of people are fed up,” Mr. Burrows said. “They got that glimmer of hope in June and July, and they’re trucking right on through. Now everyone’s used to it.”

Dr. James Volk, a vice president for Sanford Health in Fargo, N.D., where coronavirus hospitalizations have been persistently high, said he felt that fewer people were seeking medical advice about how to approach the holidays this year.

“I just think that people in general here have kind of moved on from that,” said Dr. Volk, who said he planned to stay home for Thanksgiving because of concerns about the virus.

Some authorities have called for modifications to holiday traditions.

Michigan health officials issued a holiday mask advisory on Friday — recommending that people wear a mask at indoor gatherings regardless of their vaccination status — to blunt both Covid-19 and a rising flu outbreak. Vermont officials suggested that unvaccinated children wear a mask if celebrating with their grandparents. And in New York, Gov. Kathy Hochul, while acknowledging that “no one wants to hear this again,” suggested that people avoid indoor spaces and large gatherings to curb transmission.

“We all went through this anxiety a year ago,” said Ms. Hochul, a Democrat, whose state has seen new cases increase more than 50 percent in the last two weeks. “We thought that was the last time. We declared, ‘By this time next year, I’m sure we’ll be fine. We’ll have that vaccine.’ And because there are still holdouts, we cannot declare that it’s going to be completely safe.”

Reporting was contributed by Brillian Bao in Ann Arbor, Mich.; Murray Carpenter in Unity, Maine; Dennis J. Carroll in Santa Fe, N.M.; and Jay Senter in St. Paul, Minn.