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. Of the more than 23,000 volunteers who received the vaccine, only one came down with severe Covid-19 within six months, suggesting two doses of Pfizer’s vaccine offers adequate protection, he said.