Booster protection wanes against symptomatic Omicron infections, British data suggests.

New data from Britain suggests that booster protection against symptomatic Covid caused by the Omicron variant wanes within 10 weeks.

There have not yet been enough severe cases of Omicron to calculate how well boosters protect against severe disease, but experts believe the shots will continue to provide significant protection against hospitalization and death.

“It will be a few weeks before effectiveness against severe disease with Omicron can be estimated,” the new report, from Britain’s Health Security Agency, noted. “However, based on experience with previous variants, this is likely to be substantially higher than the estimates against symptomatic disease.”

In the weeks since Omicron was discovered, multiple studies have suggested that the variant is skilled at evading the antibodies that are produced after vaccination or after infection with the coronavirus.

The new report from Britain, which included data on people who had received the AstraZeneca, Pfizer or Moderna shots, confirmed that the vaccines — both the initial two-shot series and booster doses — were less effective and waned faster against Omicron than against Delta.

Among people who received two doses of the AstraZeneca vaccine, a booster with one of the mRNA vaccines, made by Pfizer and Moderna, was 60 percent effective at preventing symptomatic disease two to four weeks after the shot. After 10 weeks, however, the Pfizer booster was just 35 percent effective. The Moderna booster was 45 percent effective at up to nine weeks. (The AstraZeneca vaccine is not authorized in the United States, but the Johnson & Johnson shot uses a similar technology.)

For people who were given three Pfizer doses, vaccine effectiveness dropped from 70 percent one week after the booster to 45 percent after 10 weeks. Pfizer recipients who received a Moderna booster, on the other hand, seemed to fare better; their vaccine regimen remained up to 75 percent effective at up to nine weeks.

The report, which was based on an analysis of about 148,000 Delta cases and 68,000 Omicron cases, also included recent data suggesting that Omicron infections are less likely to lead to hospitalizations than Delta infections. The findings should be interpreted cautiously, the agency noted, because there have still not been many Omicron cases, relatively speaking, and the people who have contracted the variant may not be representative of the broader population.

The Biden administration has been encouraging all eligible Americans to receive booster shots as Omicron spreads.

In a recent interview on WCBS-AM, a New York radio station, Dr. Anthony S. Fauci, the nation’s leading infectious disease doctor, said that officials were monitoring the effectiveness of mRNA boosters against Omicron.

“I do think it’s premature, at least on the part of the United States, to be talking about a fourth dose,” he said. Israel is weighing whether to give a fourth shot to its citizens.

Some scientists have warned against a fourth shot, noting that there is not yet evidence that it is necessary and that some immune cells might eventually stop responding to the shots if too many doses are given.