The changed calculus reflects the arrival of the more contagious coronavirus variant, first identified in Britain, which has created astonishing spikes in cases and put pressure on medical systems across the continent.

The basic scientific thinking in Europe on schools remains largely the same. When students and teachers wear masks, ventilation is good and mixing between classes is minimized, classrooms are still believed to be relatively safe and to play a small role in fueling outbreaks, compared to places such as nursing homes, prisons, food processing plants, restaurants and bars.

But the variant has increased the threat everywhere, including in schools.

“By themselves, schools are not the main problem, but it makes sense to close them when the numbers are so high that anything can have an impact on the health system as a whole,” said Celso Cunha, director of the medical microbiology unit of the Institute of Hygiene and Tropical Medicine and the Nova University of Lisbon.

Portugal, which is seeing the highest daily per capita coronavirus cases and deaths on the continent, is the latest European country to give in on schools.

Portuguese classrooms had remained open even as the government imposed a strict national lockdown this month. Prime Minister António Costa had said he wanted to “to protect and guarantee the education of this generation” and avoid the “social cost” of sending students home.

But days later, he reversed himself. The variant had expanded from 8 percent to 20 percent of coronavirus cases in Portugal within a week, and the government determined everything had to shut down.

“Despite the extraordinary efforts that schools have taken to prepare themselves,” Costa said, “in the face of the new strain, the precautionary principle requires that we proceed with a school interruption.”

CDC researchers looked to Europe’s experience in the fall to inform their conclusion that “there has been little evidence that schools have contributed meaningfully to increased community transmission.”

The fall return to classrooms was not a key driver of Europe’s second coronavirus wave, the European Centre for Disease Prevention and Control has said, and in-school transmission appeared to remain relatively uncommon, especially at the elementary school level.

But even if schools don’t play a significant role in accelerating the spread of the virus, epidemiologists say schools tend to reflect the viral situation in a community. More infections in a town mean more infections will show up in its schools, increasing the risk and the disruptions.

Additionally, since the fall, many countries in Europe have found that when everything else is locked down and students continue to go to class, schools may keep feeding cases as the virus dwindles elsewhere.

That concern is acute in the context of the variant.

“If the government reaches the stage where they are saying no one should move around, that should include children,” said Quique Bassat, a Spanish pediatrician and epidemiologist who was the coordinator of the Spanish Pediatrics Association’s working group for school reopenings.

Some prominent European virologists have been vocal about their view that keeping schools open is a major risk, including the director of the Institute of Virology at Charité Hospital in Berlin, Christian Drosten, who has inspired a rock-star-like following in Germany comparable to that of Anthony S. Fauci in the United States.

Large-scale statistical analyses that examine trends across recent studies find that children 10 and younger are about half as likely to catch the virus compared to adults. Studies are less conclusive about how likely infected children are to pass the virus on to someone else.

While initial reports suggested that children might transmit the new variant just as easily as adults, subsequent contact tracing by Public Health England documented that, as with the original virus, children are less likely to be vectors, with that effect tapering off in adolescence.

But British studies suggest the new strain is between 30 and 70 percent more contagious, an effect seen across age groups, so both students and teachers may be at greater risk.

At the Willibrord primary school outside of Rotterdam, testing after a late November outbreak found at least 123 coronavirus cases among teachers, students and family members — 15 percent of the people tested. All of the students and teachers’ cases that were sequenced were found to be the more transmissible variant, and researchers are trying to determine whether the school is responsible for the broader spread of the variant, known as B.1.1.7, in the region. But the students were not wearing masks or taking other distancing measures, factors that would have contributed to the viral spread. Dutch schools closed in mid-December, a few weeks after the Rotterdam outbreak.

Just across the border, Belgium has taken the opposite approach, shutting its schools for three weeks at the peak of a worst-in-the-world spike in November but otherwise mostly leaving lower grades in session. Secondary schools have had more distance learning.

Over the past week, confirmed cases in Belgium dropped 19 percent overall, but cases among children up to nine years old were up 88 percent. Though the increase may be partly explained by more testing in primary schools, it may also be related to students continuing to go to class. Absenteeism is rising among both teachers and students, a seeming repetition of a similar trend in October, when so many teachers were sick or quarantining that it became difficult to keep classrooms operating normally.

“It is clear that reopening schools at a moment when the equilibrium is very fragile, it can be the element that will put you at one side or the other,” said Emmanuel André, a Belgian virologist who has advised his country’s government on its pandemic response. “If you have a fragile equilibrium you always want to play on the safe side, because if it’s fragile, it can break and if breaks, it can go fast.”

But he said he still favored keeping schools open for now.

“After one year of crisis, all of these choices are balanced between public health and social well-being,” André said.

The World Health Organization, too, has advocated closing schools as a last resort.

When cases are rapidly growing despite all other measures having been taken to slow viral growth, “closing schools may unfortunately have to be that extra step, that final push” to get cases to shrink again, said Catherine Smallwood, a senior health emergency officer at the World Health Organization.

But “variant or no variant, the ambition should be to keep schools open,” she said.

Some countries in Europe still say they can safely maintain in-person education even as the more transmissible variant is set to overtake them. But many countries have resigned to closing schools.

Ireland did so this month as the country registered a wild spike in cases; reports suggest that students may not return to classrooms much before Easter.

In England, classes were back in session only briefly after winter vacation, before Prime Minister Boris Johnson announced they would close “until further notice” to slow the spread of the variant.

In Germany, schools closed in mid-December as cases rose. The federal education minister warned this month not to expect a quick return to face-to-face teaching for all students.

In Denmark — the first country in Europe to send students back last year — policymakers left schools closed after winter break even though cases were dropping, because they are trying to get cases as low as possible to better control the surge when it comes.

And Austrian students have been learning at home since November.

The increased availability of vaccines in the United States may make a difference in school reopenings, since U.S. teachers are already eligible for inoculations in some states. That would make schools safer for the adults who work in them. Europe, by contrast, is facing a vaccine shortage and hasn’t gotten beyond inoculating health-care workers, nursing homes and the elderly.

But as in Europe, momentum in the United States may shift if the variant takes hold.

Bassat, the Spanish pediatrician, said that even with the more contagious variant circulating through Spain, they had not yet seen major outbreaks at schools, which have stayed open with strict safety measures. He thought that might be a good signal for the United States.

“We are not the kings of organization,” he said. “If we’ve managed to do it without necessarily deploying big things and crazy things, then anyone can do it.”

Birnbaum reported from Riga, Latvia. Quentin Ariès in Brussels contributed to this report.