(343) Effectiveness of Maternal mRNA COVID-19 Vaccination Against COVID-19 Symptomatic Infection and Hospitalization in Infants Aged < 6 Months During the SARS-CoV-2 Omicron-Predominant Period: VENUS Study

Background: mRNA COVID-19 vaccines are not indicated in infants aged < 6 months, who are at high risk of complications from COVID-19. Transplacental transfer of antibodies against SARS-CoV-2 after maternal vaccination has been reported, but clinical evidence is limited.

Objectives: We evaluated the effectiveness of maternal mRNA COVID-19 vaccination (BNT162b2/mRNA-1273) against symptomatic infection and hospitalization for COVID-19 in infants aged < 6 months during the omicron-predominant period.

Methods: A population-based cohort study was conducted in a city (Chugoku region) of Japan using data provided by the city, including pregnancy and infant health check-up data, vaccine registry, and COVID-19 case registry. These data were linked by residential identifiers. Mothers and infants could be linked by family number. We included infants born from January 1 to December 31, 2022, and defined the infants’ birthdays as the cohort entry date (CED). We excluded infants whose mothers had a history of COVID-19 before CED. Maternal vaccination status was categorized as follows: 1) unvaccinated - never received COVID-19 vaccines before CED, 2) partially or previously vaccinated - received only first dose during pregnancy or were vaccinated only before pregnancy, or 3) fully or booster vaccinated - received a second or higher dose during pregnancy. Outcome was defined as infants’ COVID-19 outcomes (symptomatic infection/hospitalization). Follow-up started on CED and ended on the earliest of three dates: the day before the 6-month birthday, December 31, 2022, or the date of occurrence of the outcome. Rate ratios (RRs) and 95% confidence intervals (CIs) adjusted by information at CED were estimated via a Poisson regression model.

Results: We identified 895 infants (girls: 51%, median gestational age: 39 weeks, median maternal age: 31 years). Among 284 unvaccinated, 18 symptomatic infections (12.6 infants/1000 person-months [PM]) and 2 hospitalizations (1.4 infants/1000 PM) were identified; among 203 partially or previously vaccinated, 7 symptomatic infections (10.4 infants/1000 PM) and 1 hospitalization (1.4 infants/1000 PM); among 408 fully or more vaccinated, 16 symptomatic infections (8.0 infants/1000 PM) and 1 hospitalization (0.5 infants/1000 PM). Adjusted RRs and 95%CIs for symptomatic infection and hospitalization in infants were; partially or previously vs unvaccinated: 0.84 (0.35-2.01) and 1.1 (0.10-12.1), fully or booster vs unvaccinated: 0.74 (0.30-1.8) and 0.37 (0.02-6.07).

Conclusions: Although the sample size was limited, our study suggests the potential benefit of maternal full or booster vaccination during pregnancy against COVID-19 symptomatic infection and hospitalization in infants aged < 6 months during the omicron-predominant period.