The effectiveness of the Pfizer-BioNTech Covid vaccine waned over six months, but experts say the data still don't point to an immediate need for booster shots.
The study, which hasn't yet been peer-reviewed or published in a medical journal, found that the vaccine was 97 percent effective at preventing severe disease from Covid-19 for at least six months — but the effectiveness against any symptomatic illness fell from 96 percent to 84 percent in the same period, falling by about 6 percent every two months.