Researchers herein investigated the associations between immunization with a third (booster) dose of BNT162b2 vaccine (Pfizer-BioNTech) and the incidence of SARS-CoV-2 infection among healthcare workers who were previously vaccinated with a 2-dose series of BNT162b2.

• In this prospective cohort study, 1,928 health care workers in Israel were included.

• Five and 39 SARS-CoV-2 infections occurred in booster-immunized participants and booster-nonimmunized participants, respectively (incidence rate, 12.8 vs 116 per 100,000 person-days, respectively).

• Overall findings indicated reduced risk of SARS-CoV-2 infection during a median of 39 days of follow-up (adjusted hazard ratio, 0.07) in significant correlation with administration of a booster dose compared with not receiving one.