Pfizer and BioNTech release new booster trial results of their Covid vaccine.

The companies said a third shot of its vaccine sharply increased the level of antibodies against the coronavirus

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Pfizer and BioNTech release new booster trial results of their Covid vaccine.

A patient receives their booster dose of the Pfizer-BioNTech coronavirus vaccine in Southfield, Mich., on Monday.Credit…Emily Elconin/Getty Images

Aug. 25, 2021, 12:18 p.m. ET

A third shot of Pfizer-BioNTech’s Covid vaccine sharply increases the levels of antibodies against the coronavirus, the companies reported on Wednesday.

The companies were announcing the results of a study of 306 volunteers who received a booster shot about five to eight months after their second shot. Researchers found that the level of antibodies that block the coronavirus jumped more than three times higher than the level after the second dose. The side effects of a third injection were about the same as after the initial two doses, the companies said.

On Monday, the F.D.A. fully approved Pfizer-BioNTech’s two-dose vaccine for those 16 and older, making it the first to move beyond emergency use status. The companies said they are now applying to the agency for supplemental approval of the booster shot, and will submit all their supporting data by the end of this week.

Federal regulators are racing to collect and evaluate data on booster shots before the week of Sept. 20, when the Biden administration wants to begin offering them to people who were vaccinated early this year. If the F.D.A. decides additional shots are safe and effective, the administration has said it wants adults to get a third injection eight months after their second shot of the Pfizer or Moderna vaccines.

Federal health officials said last week that they believe that the potency of the Pfizer-BioNTech and Moderna vaccines wanes over time, raising the risk of infection from the highly contagious Delta variant. While data indicate that the vaccines continue to offer robust protection against hospitalization and severe disease, the officials said they fear that the situation could change without booster shots.

Pfizer executives presented an early look at their booster data on July 23, during their second-quarter earnings call. In a smaller study, they found that antibody levels dropped markedly in the months following a second dose. But those levels jumped back up after a third dose. When researchers expanded their focus to a larger group of subjects, they continued to find a strong effect from the boosters.

The participants in the new booster study were between the ages of 18 and 55.

Pfizer and BioNTech said that in addition to the F.D.A., they plan to submit their data to a peer-reviewed journal, as well as to regulatory authorities in Europe and other countries.

The administration’s booster plan does not as yet include recipients of Johnson & Johnson’s one-shot vaccine. Johnson & Johnson announced earlier on Wednesday that unlike the studies of Moderna’s and Pfizer’s vaccines, a study of 17 volunteers showed little change in their antibody levels over the course of six months.

But the study also showed that when the volunteers were given a second shot six months after their first, their antibodies against the coronavirus jumped nine times higher than the level after the first dose. Company officials said they are looking forward to discussing a potential booster strategy for their vaccine with federal health officials.

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