We conducted a randomized, placebo-controlled, observer-blinded, phase 3 trial as part of a phase 1–2–3 trial assessing BNT162b2 safety, immunogenicity, and efficacy in healthy persons 12 years of age or older. This report presents findings from 12-to-15-year-old participants enrolled in the United States, including descriptive comparisons of safety between participants in that age cohort and those who were 16 to 25 years of age and an evaluation of the noninferiority of immunogenicity in the 12-to-15-year-old cohort to that in the 16-to-25-year-old cohort. Data were collected through the cutoff date of March 13, 2021.

Eligible participants were healthy or had stable preexisting disease (including hepatitis B, hepatitis C, or human immunodeficiency virus infection). Persons with a previous clinical or virologic Covid-19 diagnosis or SARS-CoV-2 infection, previous coronavirus vaccination, diagnosis of an immunocompromising or immunodeficiency disorder, or treatment with immunosuppressive therapy (including cytotoxic agents and systemic glucocorticoids) were excluded.

The ethical conduct of the trial is summarized in the Supplementary Appendix, available with the full text of this article at NEJM.org. Additional details of the trial are provided in the protocol, available at NEJM.org. Pfizer was responsible for the trial design and conduct, data collection, data analysis, data interpretation, and writing of the manuscript that was submitted. Both Pfizer and BioNTech manufactured the vaccine and placebo. BioNTech was the regulatory sponsor of the trial and contributed to data interpretation and writing of the manuscript. All data were available to the authors, who vouch for their accuracy and completeness and for the adherence of the trial to the protocol.

Randomization was conducted with the use of an interactive Web-based response system. Participants were assigned in a 1:1 ratio to receive two intramuscular injections of 30 μg of BNT162b2 or placebo (saline) 21 days apart. For evaluation of immediate vaccine-associated reactions, participants were observed in the clinic for 30 minutes after vaccination.

Safety objectives included the assessment of local or systemic reactogenicity events, which were recorded by the participants in an electronic diary (e-diary) for 7 days after each dose. Unsolicited adverse events (i.e., those reported by the participant without e-diary prompting) and serious adverse events were also recorded from receipt of the first dose through 1 month and 6 months after dose 2, respectively.

Immunogenicity assessments (SARS-CoV-2 serum neutralization assay and receptor-binding domain [RBD]–binding or S1-binding IgG direct Luminex immunoassays) were performed before vaccination and 1 month after dose 2, as described previously.3 The immunogenicity objective was to show noninferiority of the immune response to BNT162b2 in 12-to-15-year-old participants as compared with that in 16-to-25-year-old participants. Noninferiority was assessed among participants who had no evidence of previous SARS-CoV-2 infection with the use of the two-sided 95% confidence interval for the geometric mean ratio of SARS-CoV-2 50% neutralizing titers in 12-to-15-year-old participants as compared with 16-to-25-year-old participants 1 month after dose 2. BNT162b2 immunogenicity was evaluated in participants with and those without serologic or virologic evidence of previous SARS-CoV-2 infection. Corresponding end points were the geometric mean SARS-CoV-2 neutralizing titers at baseline (i.e., immediately before receipt of the first injection) and 1 month after dose 2 and geometric mean fold rises (GMFRs) in titers from baseline to 1 month after dose 2.

The efficacy of BNT162b2 against confirmed Covid-19 with an onset 7 or more days after dose 2 was summarized in participants who did not have evidence of previous SARS-CoV-2 infection, as well as in all vaccinated participants. Surveillance for potential Covid-19 cases was undertaken throughout the trial; if acute respiratory illness developed in a participant, the participant was tested for SARS-CoV-2. Methods for identifying SARS-CoV-2 infections and Covid-19 diagnoses are summarized in the Supplementary Appendix.

The safety population included all participants who received at least one dose of BNT162b2 or placebo. The reactogenicity subset included all 12-to-15-year-old participants and a subset of 16-to-25-year-old participants (those who received an e-diary to record reactogenicity events). Safety end points are presented descriptively as counts, percentages, and associated Clopper–Pearson two-sided 95% confidence intervals, with adverse events and serious adverse events described according to terms in the Medical Dictionary for Regulatory Activities, version 23.1, for each group.

Immunogenicity was assessed in a random subset of participants in each age cohort with the use of a simple random-sample selection procedure. For immunogenicity assessments, all participants in both age cohorts were from U.S. sites. The dose 2 immunogenicity population that could be evaluated included participants who underwent randomization and received two BNT162b2 doses in accordance with the protocol, received dose 2 within the prespecified window (19 to 42 days after dose 1), had at least one valid and determinate immunogenicity result from a blood sample obtained within 28 to 42 days after dose 2, and had no major protocol deviations. Noninferiority of the immune response to BNT162b2 in 12-to-15-year-old participants as compared with that in 16-to-25-year-old participants was assessed on the basis of the geometric mean ratio of SARS-CoV-2 50% neutralizing titers. A sample of 225 BNT162b2 recipients who could be evaluated (or 280 BNT162b2 recipients overall) in each age cohort was estimated to provide 90.8% power for declaring noninferiority (defined as a lower limit of the 95% confidence interval for the geometric mean ratio of >0.67). A testing laboratory supply limitation of the qualified viral lot used for assay validation and clinical testing resulted in the trial having fewer participants than anticipated for the immunogenicity analyses. Calculations of the geometric mean ratios, geometric mean titers, and GMFRs are described in the Supplementary Appendix.

Although the formal evaluation of efficacy was to be based on the overall results obtained across all age cohorts, the statistical analysis plan specified that descriptive efficacy summaries would be provided for each age cohort (the stratification factor). The efficacy analysis for the 12-to-15-year-old cohort was planned as a descriptive analysis because the number of cases that would occur in the age subgroups was unknown. The efficacy population that could be evaluated included all eligible 12-to-15-year-old participants who underwent randomization and received two doses of BNT162b2 or placebo, received dose 2 within the prespecified window (19 to 42 days after dose 1), and had no major protocol deviations. The all-available efficacy population included all participants who received one or two doses. Vaccine efficacy was defined as 100×(1−IRR), where IRR is the ratio of the rate of a first confirmed Covid-19 illness in the BNT162b2 group to the corresponding rate in the placebo group. Two-sided Clopper–Pearson 95% confidence intervals were calculated (not adjusted for multiple comparisons). Because the number of participants who reported symptoms but were missing a valid polymerase-chain-reaction test result was small, data for these participants were not imputed in the analysis.