Health officials in many of these countries, and now in the United States, see booster shots as a way of shoring up defenses against a tenacious enemy and gaining the upper hand in the pandemic. France, for example, has mandated booster shots for those over age 65 who wish to get a health pass permitting access to public venues.

“Look what other countries are doing now about adopting a booster campaign virtually for everybody,” Dr. Anthony S. Fauci, the Biden administration’s top Covid adviser, said on Tuesday at a conference sponsored by Stat, the medical news publication. “I think if we do that, and we do it in earnest, I think by the spring we can have pretty good control of this.”

In the United States, infections have increased by 33 percent on average over the past two weeks, to 94,000 a day. The C.D.C.’s decision landed just as Americans prepare to spend the holidays with family and friends, gatherings likely to accelerate the trend.

The shots may help forestall at least some infections, particularly in older adults and those with certain health conditions. But many experts, including some who advise federal agencies, are skeptical that boosters alone can turn the tide.

The extra shots are unlikely to offer much benefit to adults under 65, who remain protected from severe illness and hospitalization by the initial immunization, the experts said.

“Overall protection remains high for severe disease and hospitalization,” said Dr. Sara Oliver, an epidemiologist at the C.D.C., told the scientific advisers meeting on Friday.

Moreover, tens of millions of Americans have not received even the first dose of a coronavirus vaccine. In the United States, as in Europe, deep pockets of vaccine-resistant adults are likely to prolong the pandemic, however well protected their neighbors may be.