Covid-19 is surging in many parts of the country in the wake of Thanksgiving, with Christmastime gatherings on the horizon.

Health authorities in some hard-hit states, like Vermont, New Jersey and Maine, say people who became infected after traveling or gathering indoors for Thanksgiving are likely adding to the Covid-19 numbers. Some 35 states have higher seven-day averages for new cases than they did before Thanksgiving, according to Johns Hopkins University data, with some of the biggest increases in the Northeast.

Epidemiologists...

Covid-19 is surging in many parts of the country in the wake of Thanksgiving, with Christmastime gatherings on the horizon.

Health authorities in some hard-hit states, like Vermont, New Jersey and Maine, say people who became infected after traveling or gathering indoors for Thanksgiving are likely adding to the Covid-19 numbers. Some 35 states have higher seven-day averages for new cases than they did before Thanksgiving, according to Johns Hopkins University data, with some of the biggest increases in the Northeast.

Epidemiologists believe colder weather, which draws people back inside where respiratory viruses can more easily spread, plays a big role.

The U.S. seven-day average for new cases is about 120,000 a day, according to Johns Hopkins, up from a recent low near 71,000 in late October, when surges triggered by the Delta variant slowed in the South before starting to hit hard in northern states. The U.S. is averaging about 1,270 daily Covid-19 deaths, which tend to lag behind cases by several weeks—about 160 ahead of the pre-Thanksgiving level.

“We have experienced, as we expected and predicted, a post-Thanksgiving spike,” New Jersey Democratic Gov. Phil Murphy said last week, regarding cases in the state. The seven-day case average there, recently about 4,170 new cases daily, has doubled since before the holiday. “There’s no other way really to look at these numbers.”

Governors and public health officials are assessing the Thanksgiving impact as cases from the new Omicron variant are growing around the U.S. However, the Delta variant overwhelmingly remains the cause of new cases, according to the Centers for Disease Control and Prevention.

Health authorities are urging people to use rapid tests to check for Covid-19 infections.

Photo: John Tully/Bloomberg News

As Christmas nears, U.S. officials have largely shied away from significant new restrictions. Instead, they are continuing to implore residents to get vaccinated, including booster shots. Health authorities are also asking people to know the vaccination status of people with whom they celebrate, urging rapid tests to check for infections and continued mask use.

“Be careful about having a whole, whole, whole bunch of people indoors at the festivities,” Kentucky Gov. Andy Beshear, a Democrat, said during a press conference. He warned about cases ramping up again there.

In New York, Democratic Gov. Kathy Hochul

said Friday that residents need to wear masks in public indoor places that don’t require vaccinations starting Monday.

Zeroing in on the Thanksgiving impact is difficult. There was a big, artificial drop in cases around the holiday as states slowed their data reporting, and then some apparent catch-up as reporting resumed, said Emily Pond, a research data analyst at the Johns Hopkins Center for Health Security.

Hospitals in many states are under rising pressure from Covid-19, including in Maine and New Hampshire, where the governors are deploying National Guard troops to bolster staffs.

Early last January, shortly after Christmas and before widespread vaccinations, Covid-19 hospitalizations peaked and the U.S. had its deadliest week with nearly 26,000 Covid-19 deaths, federal data show.

Early data already suggest that the Omicron variant may be better at evading vaccine-generated antibodies, and a lot of that has to do with what’s happening at a molecular level. WSJ’s Daniela Hernandez explains. The Wall Street Journal Interactive Edition

Health officials hope that won’t happen again, including in parts of the country with higher vaccination rates like the Northeast. More people have also acquired immune protection since last winter through Covid-19 infections.

“In many ways, we’re in a much better place than last winter,” Edward Lifshitz, medical director of the communicable disease service for New Jersey’s health department. About 69% of the state’s population is fully vaccinated, compared with closer to 61% for the U.S. overall.

Jolianne Stone,

Oklahoma’s state epidemiologist, said she hopes the state’s lower full-vaccination rate, recently 52.4%, is also enough to restrain hospitalizations and deaths there. The rate has risen about 9 percentage points since the peak of the state’s most recent hospitalization surge this summer.

Omicron worries may also encourage more people to get shots in the coming weeks, she said.

One clue about the nation’s Covid-19 trajectory comes from wastewater samples, which communities are using to scan for levels of the coronavirus that causes Covid-19. The latest data from Cambridge, Mass., firm Biobot Analytics Inc., which currently analyzes wastewater samples covering about 15 million people around the U.S., broadly show rising virus concentrations.

Given current trends, the pandemic is likely to worsen before it improves, said Philip Landrigan, an epidemiologist who directs the Program for Global Public Health and the Common Good at Boston College.

“I can’t imagine it’s going to go anywhere but up,” he said. “The message to put out to the public is please be wary and please think about the people around you.”

Write to Jon Kamp at jon.kamp@wsj.com and Ian Lovett at ian.lovett@wsj.com