How well it works: Experts continue to learn about Pfizer’s efficacy both in the laboratory and in the real world. Pfizer’s initial Phase 3 clinical data presented in December showed its vaccine to have 95% efficacy. In April,
the company announced the vaccine had 91.3% efficacy against COVID-19, based on measuring how well it prevented symptomatic COVID-19 infection seven days through up to six months after the second dose. It also found it to be 100% effective in preventing severe disease as defined by the CDC, and 95.3% effective in preventing severe disease as defined by the FDA. Another study, not yet peer-reviewed, provided
more new data that brought the efficacy number down to 84% after 6 months, although efficacy against severe disease was 97%.