Pfizer Is Studying the Effects of a Third COVID-19 Vaccine Dose.br Two doses of Pfizer's COVID-19 vaccine br have proven to be 95 effective at br preventing severe coronavirus infection.br On Feb. 25, the drugmaker announced it will be studying the effects of a third "booster" shot among those who were fully vaccinated over six months ago.br We believe that the third dose will raise the antibody response 10- to 20- fold, Pfizer CEO Albert Bourla, via NBC News.br Two age groups will be observed br in the study: 18 to 55 and 65 to 85.br The participants are among some of the first to have ever received Pfizer's vaccine in its initial clinical trials.br The third dose will be the same br as what they received a year ago.br Pfizer also intends to modify its vaccine to test against the variant from South Africa.br Pfizer CEO Albert Bourla suspects that much like the flu, "you will have to go and get your annual shot for COVID to be protected.