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Updated Covid boosters cut the infection risk from XBB.1.5 subvariant by nearly half, CDC finds

The report provides the first data on how the updated shots fare against the predominant strain in the U.S.
A health worker administers a Pfizer-BioNTech booster shot in Jackson, Miss., on Feb. 8, 2022.
A health care worker administers a Pfizer-BioNTech booster shot in Jackson, Miss., on Feb. 8.Rogelio V. Solis / AP file

The updated Covid boosters reduce the risk of Covid infection from the predominant omicron subvariant by nearly half, according to early data published Wednesday by the Centers for Disease Control and Prevention.

In adults up to age 49, the latest boosters from Pfizer-BioNTech and Moderna were 48% effective against symptomatic infection from the XBB.1.5 subvariant, the new report said. As of Jan. 21, that subvariant accounted for about 1 in 2 new cases in the U.S.

Protection was lower in older groups: The boosters were 40% effective in adults ages 50 to 64 and 43% effective in people 65 and older.

The findings are “quite reassuring,” Dr. Brendan Jackson, the head of the CDC’s Covid response, said on a call with reporters Wednesday. “These updated vaccines are protecting people against the latest Covid-19 variants.”

The Covid boosters were modified in the summer to target the BA.4 and BA.5 omicron subvariants, in addition to the original strain of the coronavirus first identified in Wuhan, China, in 2019.

BA.5 was the dominant variant in the U.S. in the fall, but now accounts for only 2% of new cases.

As of last Wednesday, only about 15% of people in the U.S. had received an updated booster, according to CDC data.

"With this data, we see there is a benefit that might convince some people to sign up and get a bivalent booster," said Dr. Peter Hotez, the co-director of the Center for Vaccine Development at Texas Children's Hospital and the dean of the National School of Tropical Medicine at the Baylor College of Medicine in Houston.

The CDC report is based on test results from more than 29,100 adults with Covid symptoms who were tested at pharmacies nationwide from Dec. 1 through Jan. 13.

People who were vaccinated but had not received the updated booster were compared to those who got the updated booster in the previous two to three months. Those who hadn't received the updated booster had their last vaccine dose about 13 months ago, Ruth Link-Gelles, who heads the CDC's vaccine effectiveness program, said on the call.

The protection provided by the booster is on par with what's typically seen with the flu vaccine. Flu vaccine effectiveness varies from season to season, but the shots reduce the risk of the flu by 40% to 60%, according to the CDC.

Dr. Greg Poland, the director of the Mayo Clinic Vaccine Research Group in Rochester, Minnesota, cautioned that the CDC's estimate on the updated boosters may be an overestimate.

People who got the updated boosters are probably "much more likely to wear masks indoors or restrain their travel or not go to indoor restaurants," he said.

He also pointed out that the CDC data doesn't capture people who were vaccinated with the updated booster but were asymptomatic, or people who were sick enough that they went to the hospital.

Hotez said that while the CDC's findings appear promising, he'd like to see data on how well the boosters perform against symptomatic infections after five or six months.

He said he'd also like to see more data on how well the updated boosters work against hospitalization.

Jackson, of the CDC, said on the call that the agency is releasing data later Wednesday that found the updated boosters reduced the risk of death from Covid by nearly thirteenfold, compared to people who are unvaccinated.

The data, he said, also found that people who got the updated booster had more than twofold lower rates of death from Covid compared to vaccinated people who did not get it.

The CDC’s report comes a day before a meeting of the Food and Drug Administration’s advisory committee that will discuss simplifying the Covid vaccination schedule.

In a document posted online Monday, the FDA proposed using the bivalent formula in all Covid vaccines moving forward, not just for booster shots.

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