Nearly 80 percent of school staff and child care workers in the United States have received at least one dose of the coronavirus vaccine, the Centers for Disease Control and Prevention said on Tuesday.

The announcement comes as the Biden administration has made an ambitious push to reopen schools and return to in-person instruction by the president’s 100th day in office. That goal has been tempered by dangerous virus variants, protests from teachers’ unions, and the fears and frustrations of students and parents.

The push to reopen schools has gathered momentum as evidence mounted that proper safety measures limited virus transmission in schools and coronavirus cases fell sharply from their January peak. Education officials and experts have cited the urgency of getting students back in classrooms before the academic year ends.

About eight million teachers, school staff and child care workers received their first vaccine dose by the end of March, according to the C.D.C., with about two million receiving their shot through the Federal Retail Pharmacy Program.

President Biden announced the program in March, urging nationwide access to vaccines for school employees and child care workers. But a hodgepodge of eligibility guidelines followed, as some states chose not to deviate from their rollout plans. By the end of March, however, K-12 educators in all states had become eligible to receive the vaccine.

While the acceleration of vaccinations among educators and staff has reduced the resistance from teachers’ unions to reopening classrooms, school systems with powerful unions, especially on the West Coast, have been slower to revert to in-person instruction.

Union resistance has led a bipartisan group of governors in several states to prod, and sometimes force, school districts to open. The result has been a major increase in the number of students who now have the option of attending school in-person, or will soon.