Effectiveness of a Third Dose of BNT162b2 mRNA Vaccine

A retrospective cohort study was carried out in a large Israeli health maintenance organization to determine vaccine effectiveness (VE) of a third dose of BNT162b2 vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Of nearly 1 million members receiving 2 doses of BNT162b2 in January-February 2021, infection rates (based on polymerase chain reaction results) were compared between those who received a third dose with those who did not during August-October 2021 (maximum, 70 days). Crude VE was 92.9% (95% confidence interval [CI], 92.6%-93.2%) and adjusted VE was 89.1% (95% CI, 87.5%-90.5%). We conclude that the third dose provides added protection against SARS-CoV-2 infection for those vaccinated 6 months ago.