The director of the Atlanta-based Centers for Disease Control and Prevention said Wednesday that American schools can safely reopen even if teachers haven’t been vaccinated against the coronavirus.

“Vaccination of teachers is not a prerequisite for safe reopening of schools,” Dr. Rochelle Walensky said. She cited CDC data showing social distancing and wearing a mask significantly reduce the spread of the virus in school settings.

White House COVID-19 coordinator Jeff Zients called on Congress to pass additional funding to ensure schools have the resources necessary to support reopening.

President Joe Biden has pledged to ensure nearly all K-8 schools will reopen for in-person instruction in the first 100 days of his administration. Biden’s top medical adviser, Dr. Anthony Fauci, said Tuesday night between 70% and 85% of Americans must get vaccinated against the coronavirus before the nation can get back to normal.

Teachers are prioritized as “essential workers” under the CDC’s vaccination plans, though many have yet to receive doses as the nation continues to face a supply shortage of the vaccine.

So far, according to the CDC, less than 2% of Americans have received both doses of the COVID vaccine.

Biden’s administration announced Tuesday that it is moving to expand access to COVID-19 vaccines, freeing up more doses for states and beginning to distribute them to retail pharmacies next week.

The push comes as pressure is building on school systems around the U.S. to reopen classrooms to students who have been learning online for nearly a year, pitting politicians against teachers who have yet to be vaccinated.

In Chicago, the rancor is so great that teachers are on the brink of striking. In California, a frustrated Gov. Gavin Newsom implored schools to find a way to reopen. In Cincinnati, some students returned to classrooms Tuesday after a judge threw out a teachers union lawsuit over safety concerns.

While some communities maintain that online classes remain the safest option for everyone, some parents, with backing from politicians and administrators, have complained that their children’s education is suffering from sitting at home in front of their computers and that the isolation is damaging them emotionally.

In Nashua, New Hampshire, the school board voted to stick with remote learning for most students until the city meets certain targets on infections, hospitalizations and tests coming back positive for the coronavirus.

Some families and their supporters have argued, too, that reopening schools would enable parents to go back to work instead of staying home to oversee their children’s education.

The CDC said in a recent study that there is little evidence of the virus spreading at schools when precautions are taken, such as masks, distancing and proper ventilation. But many teachers have balked at returning without getting vaccinated first against the scourge that has killed more than 448,000 Americans.

In California, with 6 million public school students, teachers unions say they won’t send their members into an unsafe situation.

Newsom, a Democrat, has said he will not force schools to reopen but instead wants to give them an incentive and has proposed a $2 billion plan that has met with criticism from superintendents, unions and lawmakers. It would give schools extra funding for COVID-19 testing and other safety measures if they resume in-person classes. Schools that reopen sooner would get more money.

Newsom told educators that he is willing to negotiate but that certain demands, including the call by unions to have all teachers vaccinated before school starts, are unrealistic given the shortage of shots.

“If everybody has to be vaccinated, we might as well just tell people the truth: There will be no in-person instruction in the state of California,” he said.

The biggest districts, including Los Angeles, San Diego and San Francisco, say the plan sets unrealistic rules and timelines.

“The virus is in charge right now and it does not own a calendar,” the 300,000-member California Teachers Association warned in a letter. “We cannot just pick an artificial calendar date and expect to flip a switch on reopening every school for in-person instruction.”

Biden and Republican senators have dueling proposals for stimulus packages that would distribute billions of dollars to help schools get children back into classrooms.

About 10,000 Chicago teachers and staff and 62,000 students in kindergarten through eighth grade were supposed to return to school Monday for the first time since last March. But the Chicago school system extended remote learning for two more days and called for a cooling-off period in negotiations with the teachers union.

Districtwide efforts to vaccinate Chicago’s teachers won’t begin until the middle of February.

In several states, lawmakers are advancing legislation to require more in-person learning.

An Iowa law, signed Friday by Republican Gov. Kim Reynolds, requires districts to offer full-time in-class instruction to parents who request it. Despite concerns that teachers still haven’t been vaccinated, they are set to return this month.

In North Carolina, Democratic Gov. Roy Cooper faces pressure from GOP lawmakers to reopen more schools. In South Carolina, a bipartisan push to get students back in class five days a week is underway.

“After this pandemic is over, I hope to never do another Zoom call,” said House Minority Leader Todd Rutherford, a Democrat. “I hate it. I can’t stand them. I can’t imagine being in third or fourth grade and having to stare at a screen in order to learn.”

The Associated Press contributed to this report.