(SARS-CoV-2) to include its use in adolescents 12–15 years of age (1,2). On June 2, 2021, the Israel Ministry of Health declared the availability of BNT162b2 vaccine for adolescents 12–15 years of age (3) as a 2-dose regimen, given 21 days apart. By August 26, 2021, a total of 277,218 adolescents (46.1% of those eligible) had received 1 dose of the vaccine and 187,707 (31.2%) had received 2 doses (- 1, panel A). In mid-June 2021, after a month of extremely low SARS-CoV-2 activity in Israel, 2 local outbreaks erupted (4–6). These outbreaks marked the beginning of a new widespread SARS-CoV-2 outbreak in Israel (- 1, panel B), dominated by the B.1.617.2 (Delta) variant, which accounted for 93%–99% of the sequenced viruses during July and August 2021 (7). We analyzed effectiveness of this vaccine among adolescents who had been vaccinated in the early stages of this outbreak in Israel. The study was approved by the superior ethical committee of the Israel Ministry of Health and included exemption from informed consent. The Study We performed a nationwide retrospective cohort study to estimate vaccine effectiveness against PCR-confirmed SARS-CoV-2 infections among adolescent Israel residents 12–15 years of age who had received the second vaccine dose during July 1–24, 2021. The data sources used are described in detail elsewhere (8). We estimated vaccine effectiveness and 95% CIs by using (1 – incidence rate ratio) × 100 for 1–7, 8–14, 15–21, and 22–28 days after the second vaccine dose. Incidence rate ratio denotes the ratio of the rate of PCR-confirmed SARS-CoV-2 infections in the vaccinated and unvaccinated groups. We excluded from analysis adolescents who had had a documented SARS-CoV-2–positive PCR result before the evaluation periods, regardless of their vaccination status. When several positive SARS-CoV2 test results were documented for the same person during the study period, we included only the first result in our analysis. We determined the number of unvaccinated controls for each date during July 1–24, 2021, by omitting the number of fully vaccinated adolescent Israel residents 12–15 years of age who had received the second BNT162b2 vaccine dose on a particular date from the total number of Israel residents who did not have a documented SARS-CoV-2–positive test result and had not received only a single vaccine dose by that date. We expressed the denominators of the vaccinated and unvaccinated groups in person-days. Vaccine effectiveness against severe acute respiratory syndrome coronavirus 2 infection in adolescents 12–15 years of age, by time after second dose of BNT162b2 vaccine (Pfizer-BioNTech, https://www.pfizer.com), Israel, 2021. Error bars indicate 95% CIs. Vaccine effectiveness against severe acute respiratory syndrome coronavirus 2 infection in adolescents 12–15 years of age, by time after second dose of BNT162b2 vaccine (Pfizer-BioNTech, https://www.pfizer.comExternal Link), Israel, 2021.... After administration of the second vaccine dose, crude vaccine effectiveness against laboratory-confirmed SARS-CoV-2 infection was 55.3% (95% CI 41.3%–66.0%) in the first week, 87.1% (95% CI 81.0%–91.2%) in the second week, 91.2% (87.4%–93.8%) in the third week, and 88.2% (95% CI 85.0%–90.7%) in the fourth week (Table; - 2). Vaccine effectiveness differed significantly between the first and subsequent weeks, but we found no statistically significant differences in vaccine effectiveness among the second, third, and fourth weeks. Because of the small number