Booster shots of the Pfizer-BioNTech and Moderna vaccines are not just reducing the number of infections with the highly contagious Omicron variant, they’re also keeping infected Americans out of hospitals, according to data published on Friday by the Centers for Disease Control and Prevention.

The extra doses are 90 percent effective at preventing hospitalization with the variant, the agency reported. Booster shots also reduce the likelihood of a visit to an emergency department or urgent care clinic. The data also showed that extra doses are most beneficial against infection and death among Americans ages 50 and older.

Over all, the new research indicates that the vaccines are more protective against the Delta variant than against Omicron, which lab studies have found is partially able to sidestep the body’s immune response.

“These reports add more evidence to the importance of being up-to-date with Covid vaccinations,” Dr. Rochelle Walensky, director of the C.D.C., said at a White House briefing on Friday.

While data from Israel and other countries have suggested that boosters can help prevent severe illness and hospitalization, at least in older adults, it had not been clear that the extra doses would have this effect in the United States, where patterns of vaccination and immunity differ from those elsewhere in the world.

The three studies published on Friday are by far the most comprehensive and reliable assessments of the role booster shots are playing in the U.S. pandemic. The researchers reviewed millions of cases, as well as tens of thousands of hospitalizations and deaths, as the Delta and Omicron variants each came to prominence.

“These numbers should be very convincing,” Akiko Iwasaki, an immunologist at Yale University, said of the figures released on Friday.

The detailed reports arrived along with hints that the Omicron surge may be receding. The nation is reporting 736,000 new cases daily, down from more than 800,000 last week, and hospital admissions have declined.

Yet the virus continues to spread in many states, and more than 2,000 deaths still occur on many days.

Two of the studies were published in the C.D.C.’s Morbidity and Mortality Report. In one study, researchers analyzed hospitalizations and visits to emergency departments and urgent care clinics in 10 states from Aug. 26, 2021, to Jan. 5, 2022.

Vaccine effectiveness against hospitalization with the Omicron variant fell to just 57 percent in people who had received their second dose more than six months earlier, the authors found. A third shot restored that protection to 90 percent.

The second study looked at nearly 10 million Covid cases and more than 117,000 associated deaths recorded at 25 state and local health departments between April 4 and Dec. 25, 2021.

Cases and deaths were lower among people who had received a booster dose, compared with those who were fully vaccinated but did not receive a booster, and much lower than the rates seen among unvaccinated people, the researchers reported.

Booster doses provided much larger gains in protection among people ages 65 and older, followed by those ages 50 to 64, the study found. The researchers did not offer data on the benefits of the shots in younger people.

In the third study, published in the journal JAMA, data from more than 70,000 people who sought testing showed that a third dose provided more protection against symptomatic infection than two doses or none. Full vaccination and boosters were less protective against the Omicron variant than against Delta.

On Thursday night, the C.D.C. published additional data on its website showing that in December, unvaccinated Americans 50 years and older were about 45 times more likely to be hospitalized than those who were vaccinated and got a third shot.

Together, the studies make a powerful case that boosters are a valuable defense against Omicron. Yet less than 40 percent of fully vaccinated Americans who are eligible for a booster shot have received one.

It’s too soon to know whether protection from the extra shots might wane, noted Natalie Dean, a biostatistician at Emory University.

“We just have to recognize that all these estimates of Omicron third-dose protection are going to be people who are pretty recently boosted,” she said.

The C.D.C. now recommends booster shots for everyone 12 years and older, five months after getting two doses of the mRNA vaccines made by Pfizer-BioNTech and Moderna, or two months after a single dose of the Johnson & Johnson vaccine.

When debating booster shot recommendations for all American adults, scientific advisers to the Food and Drug Administration and the C.D.C. repeatedly bemoaned the lack of data specific to the United States.

There are differences between Israel and the United States — for example, in the way Israel defines severe illness — that made it challenging to interpret the relevance of Israeli data for Americans, they said.

Some members of the Biden administration supported the use of booster doses even before the scientific advisers of the agencies had a chance to review the data from Israel. Federal health officials intensified the boosters-for-all campaign after the arrival of the Omicron variant.

The usefulness of booster shots in Americans younger than 50 was a topic of vigorous debate in the fall. Several experts argued at the time that third shots were unnecessary for younger adults because two doses of the vaccine were holding up well.

Some of those experts remained unconvinced by the new data.

It was clear even months ago that older adults and those with weakened immune systems would benefit from extra doses of the vaccine, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and a member of the F.D.A.’s vaccine advisory committee.

But “where is the evidence that a third dose benefits a healthy young person?” he asked.

“If you’re trying to stop the spread of this virus, vaccinate the unvaccinated,” he added. “We keep trying to further protect the already protected.”

But other experts changed their minds in favor of boosters with the arrival of the highly contagious Omicron variant. Even if two doses were enough to keep young people out of hospitals, they said, a third dose could limit the spread of the virus by preventing infections.

“They’re both data-driven, legitimate positions,” said John Moore, a virologist at Weill Cornell Medicine in New York.

But at this point, the debate is over: “We are using boosters in everyone, and that’s what’s happening.”