Adults in all U.S. states are now eligible for vaccination, hitting Biden’s target. Half have had at least one dose.

Even if there is a link between the vaccine and the clotting disorder, the risk is exceedingly low, experts say.

Still, Dr. Mani said the pause was likely to harden the hesitancy of some Americans to get vaccinated.

At the same time, with the virus resurgent, public health experts are warning Americans not to let their guards down. The United States is averaging more than 67,000 new cases a day over the past seven days, up from over 54,000 a month ago, according to a New York Times database.

“Seventy thousand cases a day is not acceptable. We have to get that down,” said Barry Bloom, a research professor and former dean of the Harvard T.H. Chan School of Public Health. He said more vaccinations would help, but people must remain vigilant about wearing masks and social distancing.

At its current pace, the United States will vaccinate 70 percent of its population by mid-June. But vaccine hesitancy could slow progress toward herd immunity, which will also depend on vaccinating children.

“We’re making tremendous progress, but we’re still in the race against this virus, and we need to vaccinate tens of millions more Americans,” Mr. Biden said on Sunday. “We could have a safe and happy Fourth of July with your family and friends in small groups in your backyard. That’s going to take everyone doing their part. Get vaccinated.”

Pfizer announced this month that it had applied for an emergency use authorization to make children ages 12 to 15 eligible for its vaccine. Moderna is expected to release results from its trial in young teenagers soon, and vaccinations in this age group could begin before school starts in the fall.