Booster shots in adults who received the Pfizer-BioNTech vaccine were highly effective at preventing symptomatic Covid-19 breakthrough infections, Pfizer announced on Thursday.

The company said that out of more than 5,000 Pfizer-BioNTech vaccine recipients enrolled in its study who received a booster shot, only five later developed symptomatic disease, compared with 109 people among a similar group that received a placebo instead of a booster dose.

The news arrived as an advisory committee to the Centers for Disease Control and Prevention debates whether Americans should receive booster shots of the Moderna and Johnson & Johnson vaccines.

The company claimed the findings came from the first randomized efficacy trial of booster shots. But the results, announced in a news release, have not been peer-reviewed or published in a medical journal.

Last month, the Food and Drug Administration authorized Pfizer-BioNTech booster shots for people 65 and over, people who are at high risk of severe Covid-19, and those who are at elevated risk of exposure because of where they work or live. That decision was based on limited effectiveness data.

The new findings appear to bolster proof that booster shots are highly effective, though the trial participants were only followed for a median period of two and a half months after receiving the booster.

“These important data add to the body of evidence suggesting that a booster dose of our vaccine can help protect a broad population of people from this virus and its variants,” said Dr. Ugur Sahin, founder and chief executive of BioNTech.

The results will be shared with the F.D.A. and its European equivalent, the European Medicines Agency, as well as other international regulatory agencies, according to Albert Bourla, Pfizer’s chief executive.

The randomized controlled trial of the booster included more than 10,000 participants aged 16 and older, half of whom received a booster that contains the same amount of vaccine as each of the two primary doses, and half of whom received a placebo.

The booster was given an average of 11 months after the initial regimen, and participants were monitored for symptoms of Covid that developed between a week and 2.5 months after the booster, on average.

Stratified analyses showed the relative efficacy rate of 95.6 percent for the boosters was consistent regardless of age, sex, race, ethnicity or chronic medical conditions.

Slightly more than half of the participants were between 16 and 55 years old, and just under one quarter were 65 or older. The companies said that they had not identified any new side effects or safety concerns during the trial.



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