Effectiveness of BNT162b2 Vaccine Booster against SARS-CoV-2 Infection and Breakthrough Complications, Israel

T he mass severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) BNT162b2 (Pfizer-BioNTech, https://www.pfizer.com)vaccination campaign in Israel was associated with a decline in the number of SARS-CoV-2 infections, hospitalizations, and deaths, reaching a nadir by mid-May 2021 (1).However, beginning the third week of June 2021, a new rise in the number of SARS-CoV-2 cases was observed, including cases among fully vaccinated persons (1,2).Waning humoral immune response after the second vaccine dose was then found to be associated with increased incidence of SARS-CoV-2related infections, hospitalizations, and deaths caused primarily by the B.1.617.2 (Delta) variant (3).In response to the increasing illness and deaths, the Israel Ministry of Health (MOH) recommended a third (booster) BNT162b2 vaccine dose for persons for whom at least 5 months had passed after receiving the second vaccine dose (4).The elderly and other high-risk groups were prioritized at first (4), and other age groups were added rapidly thereafter (5).We estimated the booster dose vaccine effectiveness (VE) against SARS-CoV-2 infection and the rate reduction of complications in breakthrough coronavirus disease (COVID-19) cases after the BNT162b2 booster dose in persons >16 years of age, by age group, for up to 20 weeks after receipt of the booster dose. Methods Study DesignWe conducted a retrospective longitudinal cohort study using 2 MOH national repositories: the COVID-19 vaccine repository and the SARS-CoV-2 test repository.The national COVID-19 vaccine repository includes vaccine type, vaccine lot number, and date of dose administration for each person vaccinated in Israel.The national SARS-CoV-2 PCR test database includes the results of each test performed, the date of testing, and the date results were obtained for each person.It also includes the date of hospitalization, severity of illness, and date of death of persons with COVID-19, if applicable.Personal identifiers such as unique personal identity number, age, and sex of each person registered in the repositories (because of PCR testing or vaccination) are included in both databases.