Dr. Anthony Fauci, the nation’s chief epidemiologist, said Tuesday he supports efforts to mandate COVID-19 vaccines for teachers, one of the strongest signals of support for such measures from a federal official leading the nation’s response to the pandemic.

Such mandates would not be made at the federal level, said Fauci, President Joe Biden’s top medical adviser, in an appearance on MSNBC. But local requirements may be useful as schools reopen amid the spread of the more-contagious Delta variant of COVID-19, he said.

“I’m going to upset some people on this, but I think we should,” Fauci said of vaccine requirements for teachers. “I mean we are in a critical situation now. We’ve had 615,000-plus deaths, and we are in a major surge now as we are going into the fall, into the school season. This is very serious business.”

He made his comments in response to a question about American Federation of Teachers President Randi Weingarten saying over the weekend that she personally supports educator vaccine requirements.

Some local union chapters have pushed back against possible vaccine mandates as an increasing number of state and local leaders announce requirements for public employees, including teachers in some cases. But others have changed their position as case numbers and hospitalizations rise in many areas.

On Tuesday, the union for Washington, D.C., schools became the latest to support such a requirement. Later, Mayor Muriel Bowser announced a that the city’s teachers would be required to be vaccinated and that that those who were exempt from the policy would be tested weekly for the virus.

Federal officials prioritized teachers for early vaccine doses last winter as part of their strategy to return students to in-person learning. The vaccinations not only protect those who receive them, but they can also help lower the likelihood of transmission for those around them, including children under age 12, who are too young to be vaccination, epidemiologists have said.

By April, the Centers for Disease Control and Prevention announced that about 80 percent of teachers, school staff, and child-care workers had received at least one vaccine dose. Both major national teachers’ unions now say about 90 percent of their members are vaccinated.

But there are parts of the country where vaccination rates are lower across populations, including among educators in some districts. Public health officials in those areas have said more adults should be vaccinated to help ensure safe in-person learning with fewer interruptions for quarantines.

However, some states, including Arkansas, Florida, and Texas, have prohibited state and local vaccine requirements for public workers, including teachers.

Some public health experts have urged caution on talk of vaccine mandates, saying they may stir up anger among anti-vaccination organizers. But more businesses and public officials have set such requirements in the last month, expressing concern about the Delta variant.

The three vaccines in use in the United States have emergency authorization from the Food and Drug Administration. Some officials have said they will wait to consider mandates until after the vaccines receive full, permanent approval.

Fauci said he expects that approval to come “very soon.”