According to data released Friday by the Centers for Disease Control and Prevention.
Additional doses are 90% effective against hospitalization with the variant, the agency reported. Booster shots also reduced the likelihood of a visit to an emergency department or urgent care clinic. Extra doses were most effective against infection and death in Americans ages 50 and older, the data showed.
Overall, the new data show that the vaccines were more protective against the Delta variant than against Omicron, which lab studies show is partially able to bypass the body’s immune response.
It is generally believed that booster shots keep people from getting infected, at least for a while. Data from Israel and other countries have also suggested that boosters can help prevent serious illness and hospitalizations, especially among older people.
“Data from other countries also showed a significant benefit in getting the booster, but this really shows it in the United States,” Akiko Iwasaki, an immunologist at Yale University, said of the numbers released Friday. “These numbers should be very compelling.”
On Thursday evening, the CDC released additional data showing that in December, unvaccinated Americans 50 and older were about 45 times more likely to be hospitalized than those who were vaccinated and received a third shot.
Yet less than 40% of fully vaccinated Americans are eligible for a booster shot to have received one.
Friday’s findings are based on three new studies conducted by the CDC In one study, researchers analyzed hospitalizations and visits to emergency departments and urgent care clinics in 10 states from August 26, 2021 to January 5 2022.
Vaccine effectiveness against hospitalization with the Omicron variant fell to just 57% in people who had received their second dose more than six months earlier, the authors found. A third shot restored this protection to 90%.
It’s unclear whether protection from boosters might also wane like after two injections, noted Natalie Dean, a biostatistician at Emory University.
“We just have to recognize that all of these estimates of protection per third dose of Omicron will be for people who have recently been boosted,” she said. “We wonder about the durability of the boosters themselves.”
During the debate over booster recommendations for all US adults, scientific advisers to the Food and Drug Administration and the CDC repeatedly lamented the lack of US-specific booster data.
There are differences between Israel and the United States — for example, in how Israel defines serious illnesses — that have made it difficult to interpret the relevance of Israeli data to Americans, they said.
Some members of the Biden administration have backed the use of booster doses even before the agencies’ scientific advisers had a chance to review Israel’s data. Federal health officials stepped up their campaign of recalls for all after the arrival of the Omicron variant.
CDC now recommends booster shots for everyone from 12 years old, five months after receiving two doses of the mRNA vaccines made by Pfizer-BioNtech and Moderna, or two months after a single dose of the Johnson & Johnson vaccine.
The usefulness of booster shots in Americans under 50 was the subject of vigorous debate in the fall. Several experts argued at the time that third shots were not necessary for young adults because two doses of the vaccine held together well.
Some of these experts are still unconvinced by the new data.
It was clear even months ago that the elderly 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 fellow of the FDA Vaccine Advisory Committee. .
But “where is the evidence that a third dose benefits a healthy young person?” He asked.
“If you are trying to stop the spread of this virus, vaccinate the unvaccinated,” he added. “We continue to try to further protect those who are 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, they said, a third dose could limit the spread of the virus by preventing infections.
“These are both legitimate data-driven posts,” said John Moore, a virologist at Weill Cornell Medicine in New York. But at this point, the debate is over: “We use boosters in everyone, and that’s what happens.”