The efficacy of the Covid-19 vaccine developed by Pfizer and BioNTech fell from 96% to 84% over six months, according to data released Wednesday, a decline that could fuel Pfizer’s case that a third dose will eventually be required.
The data, released in a preprint that has not been reviewed by outside scientists, suggest the vaccine was 91% effective overall at preventing Covid-19 over the course of six months.
In the ongoing study, which enrolled more than 44,000 volunteers, the vaccine’s efficacy in preventing any Covid-19 infection that causes even minor symptoms appeared to decline by an average of 6% every two months after administration. It peaked at more than 96% within two months of vaccination and slipped to 84% after six months.
Against severe disease, which includes people with low blood oxygen levels or who are hospitalized, the overall efficacy of the vaccine was 97%.
To Paul Offit, a pediatrician and vaccine expert at Children’s Hospital of Philadelphia, the results are “very reassuring.” The potential need for booster shots is tied to the number of fully vaccinated people who develop severe disease, Offit said. That number is just 3% after six months, suggesting two doses of Pfizer’s vaccine offers adequate protection, he said.
While the data suggest Pfizer and BioNTech’s vaccine provides lasting protection against the worst symptoms of Covid-19, the paper leaves open the possibility that booster doses will eventually be necessary to curtail infection. If the vaccine’s efficacy continues to decline at the rate observed in the paper, it would fall below the 50% threshold — a benchmark for vaccine utility — within 18 months of vaccination.
That would support Pfizer’s contention that two doses of the vaccine won’t be enough to provide long-term protection. Federal authorities have maintained that people who have been fully vaccinated don’t need booster doses yet but that they are continuing to look at new data.
Moderna’s vaccine was 90% effective against symptomatic Covid-19 and 95% effective against severe disease after six months, the company said in April. Johnson & Johnson has not yet disclosed six-month efficacy data.
The Pfizer study, which enrolled volunteers in Europe and the Americas, doesn’t address whether the vaccine might be less effective against the fast-spreading Delta variant. Pfizer and BioNTech have conducted lab studies suggesting the vaccine should be able to neutralize the variant, but there is no large-scale clinical data to confirm that conclusion.
Just over half of the study’s participants were male. About 82% of participants were white, 9.5% were Black, 4.3% were Asian, 1% were Native American. Roughly 26% of volunteers identified as Hispanic.