In the new study, the researchers followed the volunteers for six months after vaccination, up to March 13. Over the entire period, the researchers estimated, the vaccine’s efficacy was 91.5 percent against symptomatic Covid-19. (The study did not measure the rate of asymptomatic virus infections.)
But within that period, efficacy did gradually drop. Between one week and two months after the second dose, the figure was 96.2 percent. In the period from two to four months following vaccination, efficacy fell to 90.1 percent. From four months after vaccination to the March cutoff, the figure was 83.7 percent.
Those figures still describe a remarkably effective vaccine, however, and may not convince critics that booster shots are widely needed.
The study comes on the heels of data from Israel suggesting that the Pfizer-BioNTech vaccine’s protection may be waning there. But experts have pushed back against a rush to approving a booster there. The data have too many sources of uncertainty, they say, to make a precise estimate of how much effectiveness has waned. For example, the Delta-driven outbreak hit parts of the country with high vaccination rates first and has been hitting other regions later.
“Such an analysis is still highly uncertain,” said Doron Gazit, a physicist at Hebrew University who analyzes Covid-19 trends for the Israeli government.
Earlier on Wednesday, Pfizer reported that a third dose of its vaccine significantly increases blood levels of antibodies against several versions of the virus, including the Delta variant.
Results were similar for antibodies produced against the original virus and the Beta variant, which was first identified in South Africa. Pfizer and BioNTech expect to publish more definitive research in the coming weeks.