SARS-CoV-2 Vaccination Before and During Pregnancy and Prevention of Infant COVID-19 Infection

BACKGROUND: SARS-CoV-2 vaccination during pregnancy reduces COVID-19 risk in infancy. Whether maternal vaccination before pregnancy similarly protects infants against COVID-19 is unknown. METHODS: We examined the effectiveness of maternal messenger RNA SARS-CoV-2 vaccination before and during pregnancy in preventing COVID-19 in infants aged 0-6 months born between July 1, 2021, and June 30, 2023, at Kaiser Permanente Northern California. Maternal vaccination status was categorized as vaccinated during pregnancy, during a prepregnancy interval (0-<3, 3-<6, 6-12, and >12 months prepregnancy), or unvaccinated. Secondary analyses examined the effectiveness of vaccination by trimester. COVID-19 was defined by positive SARS-CoV-2 PCR result or diagnostic code, and COVID-19-related hospitalizations were confirmed by medical record review. Vaccine effectiveness (VE) was examined by Cox regression models adjusted for maternal and infant characteristics, calculated as (1 - hazard ratio) × 100%. RESULTS: Among 78 644 infants, 3648 (4.6%) had COVID-19 infection and 76 (0.1%) experienced COVID-19-related hospitalization before age 6 months. For the 4 prepregnancy vaccination intervals, estimates for VE against COVID-19 infection ranged from -14.9% (95% CI, -32.8% to 0.5%) for less than 3 months prepregnancy to 23.6% (-14.3% to 49%) for more than 12 months prepregnancy. VE anytime during pregnancy was 7.5% (-2% to 16.2%) against infant COVID-19 infection and 52.9% (11.1%-75.1%) against infant COVID-19-related hospitalization. Effectiveness of third-trimester vaccination was 19.2% (8.6%-28.6%) against infant COVID-19 infection and 64.6% (12.3%-85.7%) against infant COVID-19-related hospitalizations. CONCLUSION: Maternal vaccination during the third trimester was protective against infant COVID-19 infections. Maternal vaccination during pregnancy, particularly during the third trimester, was effective against infant COVID-19-related hospitalizations. Vaccination before pregnancy did not protect infants.