Pfizer announced Friday that its updated coronavirus booster shot targeting the omicron variant succeeded in strengthening the body’s army of disease-fighting antibodies, even as questions persist about the vaccine’s continued effectiveness against a swarm of later-generation versions of omicron.

The findings reinforce the public health message that the new bivalent boosters, which were rebooted in September to match newer omicron subvariants, could help protect people this fall and winter.

“Based on all the data we now have in hand, we have confidence in the bivalent covid-19 vaccines and their ability to create better protection against COVID-19, including these currently circulating variants, than the original vaccine would have,” said Peter Marks, director of the Food and Drug Administration’s Center for Biologics Evaluation and Research. He urged the public to consider getting the updated booster ahead of Thanksgiving.

Pfizer presented the data in a news release, and it has not been peer reviewed or published.

The news release does not directly address a concern raised by small studies over the past week that immune-evading variants that have recently emerged and are quickly taking over appear to be far better at eluding the virus-fighting antibodies generated by the booster.

One of the most threatening new variants gaining ground in the United States, BQ.1.1, is related to BA.5, a component of the new booster, so Marks said he was hopeful the bivalent vaccine would hold up, particularly against severe illness and hospitalization.

The new data also adds to, but does not resolve, an ongoing scientific debate about the extent to which the new boosters are better than the old boosters. This remains a critical issue for scientists to sort out as they design a long-term vaccine strategy against the virus. But this technical debate is largely separate from the public health issue of whether to get a booster. The old boosters have been replaced by the bivalent shots, with the exception of the Novavax vaccine, and the Pfizer data shows the new shots provide a meaningful boost to antibodies, a key line of immune defense.

The new vaccine strengthened omicron-blocking antibody levels over baseline in adults of all ages – a 9.5-fold increase in adults younger than 55 and a 13.2-fold increase in older adults.

In adults older than 55, the new booster provoked a stronger immune response against BA.4 and BA.5 than the original booster, although it was unclear whether the difference, measured in laboratory tests, would provide a meaningful difference in how well people were protected.

It was unclear if the new shot also provoked a stronger response in younger adults than the old booster, because there was no comparison group of younger adults that received the original shot.

A month after vaccination, the new booster triggered a fourfold increase in omicron-blocking antibody levels against the BA.4/BA.5 version of omicron compared with the old booster in older adults. That is an advantage, but scientists have varying opinions on whether it will make a difference in whether people are likely to fall ill.

Some scientists have argued that a 10-fold difference would probably be clinically meaningful. Marks said a fourfold increase in antibodies over the original vaccine may protect people better against symptomatic infection and increase the durability of protection because antibodies, which decay over time, would be receding from a higher initial level.

The bivalent boosters clearly strengthen immunity and are a critical tool going into a possible winter surge, but they have also raised questions about how to design the best vaccine strategy for a population that has a varied history of infection and vaccination going forward.

“There needs now to be a serious rethink about national covid vaccine policy. Chasing variants with booster redesigns clearly isn’t a sustainable solution,” said John P. Moore, a professor of microbiology and immunology at Weill Cornell Medicine who has been critical of the formulation chosen for the omicron-specific booster.

Adoption of the updated boosters has been disappointing so far. According to the Centers for Disease Control and Prevention, only 10 percent of people 18 and older have received a new booster. Low booster coverage is especially worrisome among adults 65 and older, who are at increased risk of coronavirus complications because of age. About 23 percent of people in that age group have received the new boosters.

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