- A third dose of Pfizer and BioNTech's coronavirus vaccine restored the shot's strong protection against COVID-19 in a large randomized trial, the companies said Thursday, offering the most substantial clinical evidence to date in support of booster shots.
- In the Phase 3 study, more than 10,000 adults 16 years or older were randomized to receive a booster or a placebo. All had previously received two doses of Pfizer's vaccine. Results showed there were five cases of symptomatic COVID-19 in the booster group, compared to 109 among those who received placebo — a relative efficacy rate of 95.6%.
- Pfizer said the results were consistent regardless of age, sex, ethnicity or underlying health condition. The company will use the results to seek full approval of boosters in the U.S., where a third shot is only available to certain groups under an emergency authorization.
The debate among public health officials and scientists about COVID-19 booster shots has largely centered on the the quality of evidence supporting broad use across the general public.
Pfizer, Moderna and Johnson & Johnson each sought clearance of booster doses based on results showing they could substantially raise levels of antibodies against the coronavirus. Data from Israel, where boosters have been rolled out broadly, as well as population-level data from the U.S., had also indicated waning protection from initial vaccinations.
Across a series of meetings, advisers to the Food and Drug Administration grappled with some of the uncertainties those data presented. The benefits of an additional shot in younger and healthier adults were less clear-cut than in the elderly. And safety data, particularly regarding a type of heart inflammation called myocarditis that's been observed in rare cases with Pfizer's and Moderna's vaccines, were sparse. (Myocarditis is also associated with COVID-19.)
The incomplete data package led the advisers to rebuff Pfizer's initial attempt to win standard approval for broad use of boosters. The agency instead narrowed the groups eligible for a booster and cleared the request through an emergency authorization, a blueprint the FDA subsequently followed with Moderna's shot.
The summary trial results released by Pfizer on Thursday could go a long way toward resolving some of those lingering questions, proving that boosting fully vaccinated individuals can prevent symptomatic COVID-19.
The 95.6% relative efficacy rate was observed in people a median of 53 years old, with more than half of the volunteers between 16 and 55 years of age — groups currently ineligible for a Pfizer booster, unless they have certain health conditions or are frequently exposed to the virus because of their jobs. Importantly, the shot's safety profile was "consistent with other clinical safety data" for the vaccine, Pfizer said.
The data disclosed, however, are only summary results. It's unclear how many severe COVID-19 cases were prevented, if any. The durability of a boost isn't known either, as the shot's effectiveness was measured after a median follow-up of two-and-a-half months.
Additionally, the efficacy Pfizer reported was in study participants who hadn't been previously infected with the coronavirus. Pfizer didn't say how effective boosters were for those who had recovered from a prior infection, which was part of another study goal.
Pfizer also didn't specifically detail rates of myocarditis overall, or across different age groups. Myocarditis is more commonly seen in younger men.
More convincing data of booster shots' utility doesn't help address the questions of vaccine equity that have also shadowed debate over their use in the U.S. and other wealthy nations, which disproportionately hold the bulk of the world's vaccine supply.
Though BioNTech and Moderna have committed to building vaccine manufacturing facilities in Africa, those efforts won't have an immediate impact on a continent that remains less than 5% vaccinated. The global vaccine alliance COVAX has underperformed expectations as well, in part because of a heavy reliance on Novavax, which has reportedly struggled to mass-produce its shot.