Booster Shots Instrumental in the fight against Omicron, C.D.C. show data

Booster shots of the Pfizer-BioNTech and Moderna vaccines not only prevent infections with the highly contagious Omicron variant — they also keep infected Americans from ending up in the hospital, according to data released Friday by the Centers for Disease Control and Prevention.

The extra doses are 90 percent effective against hospitalization with the variant, the agency reported. Booster shots also reduced the likelihood of a visit to an emergency room or clinic. The extra doses were most effective at preventing infection and death in Americans age 50 and older, the data showed.

Overall, the new data shows that the vaccines were more protective against the Delta variant than against Omicron, which laboratory studies have shown can partially bypass the body's immune response.

It's generally accepted that booster shots keep people from getting infected, at least for a while. Data from Israel and other countries also suggest that booster shots can help prevent serious illness and hospitalizations, particularly among older adults.

"Data from other countries have also shown a significant benefit from the booster, but this really shows it in the US," Akiko Iwasaki, an immunologist at Yale University, said of the numbers released on Friday. "These numbers should be very compelling."

On Thursday evening, the C.D.C. released additional data showing that in December, unvaccinated Americans age 50 and older were about 45 times more likely to be hospitalized than those who were vaccinated and received a third shot.

Yet fewer than 40 percent of fully vaccinated Americans who are eligible for a booster shot have received one.

Friday's findings are based on three new studies led by the C.D.C. In one study, researchers analyzed hospitalizations and visits to emergency departments and emergency clinics in 10 states from August 26, 2021 to January 5, 2022.

The vaccine's effectiveness against hospitalization with the Omicron variant dropped to just 57 percent in people who had received their second dose more than six months earlier, the authors found. A third shot restored this protection to 90 percent.

It's unclear if protection from the boosters might also wear off, as it did after two shots, noted Natalie Dean, a biostatistician at Emory University.

"We just have to acknowledge that all of these Omicron third-dose protection estimates will be people who have only recently been boosted," she said. "We wonder how durable the boosters themselves are."

In discussing booster recommendations for all American adults, scientific advisers from the Food and Drug Administration and the C.D.C. repeatedly lamented the lack of booster shot data specific to the United States.

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

The coronavirus pandemic: important things to know

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

The CDC now recommends booster shots for everyone ages 12 and older, five months after receiving two doses of the Pfizer-BioNtech and Moderna mRNA vaccines, or two months after a single dose of the Johnson & Johnson vaccine.

The usefulness of booster shots in Americans under 50 was a subject of heated debate this fall. Several experts at the time argued that third shots were unnecessary for younger adults because two doses of the vaccine lasted well.

Some of these experts were not convinced by the new data.

It was clear months ago that older adults and those with compromised immune systems would benefit from additional 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 FDA's Vaccine Advisory Committee.

But "where is the evidence that a third dose will benefit a healthy young person?" he asked.

"If you're trying to stop the spread of this virus, vaccinate the unvaccinated," he added. "We continue to try to further protect what is already protected."

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

"Both are data-driven, legitimate positions," said John Moore, a virologist at Weill Cornell Medicine in New York. But that's where the debate ends: "We use boosters in everyone, and that's what happens."