Efficacy of a Fourth Dose of Covid-19 mRNA Vaccine against Omicron

A Fourth Dose of mRNA Vaccine in Health Care Workers Health care workers in Israel were given a fourth dose of BNT162b2 or mRNA-1273 vaccine during a period of omicron-variant predominance. Levels of antibodies to SARS-CoV-2 had waned 5 months after a third dose, and the fourth dose boosted antibodies to the maximum level observed after the third dose but no higher. Vaccine efficacy was 31 to 43% against symptomatic disease.