Effectiveness of BNT162b2 Vaccination During Pregnancy in Preventing Hospitalization for Severe Acute Respiratory Syndrome Coronavirus 2 in Infants

OBJECTIVE: To assess the clinical effectiveness of the BNT162b2 vaccine during pregnancy in preventing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) hospitalizations of infants. STUDY DESIGN: A retrospective, multicenter, 1:3 case-control (test-negative) study. Symptomatic hospitalized infants less than 6 months of age, with a positive SARS-CoV-2 polymerase chain reaction test between January 3, 2021, and March 11, 2021, were matched by age and time to negative controls, hospitalized with symptoms compatible with SARS-CoV-2 infection. Mothers were defined as fully vaccinated who received 2 doses of BNT162b2 with the second given 2 weeks to 6 months before delivery; or partially vaccinated, if they received only 1 dose or 2 doses with the second given more than 6 months or less than 2 weeks before delivery. Severe SARS-CoV-2 was defined as a need for assisted ventilation. RESULTS: We matched 116 SARS-CoV-2 positive infants with 348 negative controls with symptoms compatible with SARS-CoV-2 infection. The effectiveness of fully vaccinated mothers was 61.6% (95% CI, 31.9-78.4) and the effectiveness of partially vaccinated mothers was not significant. Effectiveness was higher in infants 0-2 vs 3-6 months of age. The effectiveness (57.1%; 95% CI, 22.8-76.4) was similar when excluding mothers who were infected with SARS-CoV-2 during pregnancy. The OR of severe infection in infants born to unvaccinated vs fully vaccinated mothers was 5.8. CONCLUSIONS: At least 2 doses of BNT162b2 vaccine administered during the second or third trimester of pregnancy had an effectiveness of 61.6% in decreasing hospitalization for SARS-CoV-2 infection in infants less than 6 months of age.