1. In this retrospective cohort study, maternal vaccination with an mRNA COVID-19 vaccine series during pregnancy was highly effective against delta and moderately effective against omicron infection in infants younger than six months of age.
2. Maternal vaccination with an mRNA COVID-19 vaccine series during pregnancy was highly effective against delta and moderately effective against omicron infection requiring hospital admission in infants younger than six months of age.
Evidence Rating Level: 2 (Good)
Most cases of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) in infants present with mild or no symptoms, but rates of admission to hospital and severe illness have been higher in infants compared with older children, especially during the first month of life. COVID-19 vaccines are effective against severe infection but are not yet licensed in infants less than 6 months old. Passive immunity for infants through the transfer of maternal antibodies after vaccination is well-established for preventing infections, and emerging evidence suggests that SARS-CoV-2 maternal vaccination may reduce the risk of SARS-CoV-2 infection and hospitalization in infants. This population-based, test-negative design study aims to evaluate the effectiveness of maternal vaccination with the mRNA COVID-19 vaccine series during pregnancy against delta and omicron SARS-CoV-2 infection and hospital admission of infants during their first six months of life. Included were infants under 6 months of age who were born between May 2021 and March 2022 and who were tested for SARS-CoV-2 between May 2021 and September 2022. Data was collected from ICES, an independent, non-profit research institute in Ontario with databases including clinical, laboratory, billing, and sociodemographic data. The primary outcome studied was a positive real-time polymerase chain reaction (PCR) test on a respiratory specimen. In total, 8809 infants met the inclusion criteria, including 99 delta cases compared to 4365 controls, and 1501 omicron cases compared to 4847 controls. Vaccine effectiveness for the primary vaccine series was 95% (95% confidence interval (CI), 88% to 98%) against delta infection and 45% (95% CI, 37% to 53%) against omicron infection. Primary plus booster vaccine effectiveness was 73% (95% CI, 61% to 80%) against omicron infection. In addition, 29 (29%) of 99 infants were admitted to the hospital because of a delta infection, and 330 (22%) of 1501 infants were admitted to the hospital because of an omicron infection. Vaccine effectiveness for the primary vaccine series was 97% (95% CI, 75% to 100%) for the delta variant and 53% (95% CI, 39% to 64%) for the omicron variant against hospital admission. Overall, the findings from this study suggest high vaccine effectiveness for maternal vaccination with the primary mRNA covid-19 vaccine against delta and omicron infections, as well as admission to hospital in infants younger than six months of age. A limitation of this study is the effect of potential confounders, such as breastfeeding, that could have differed between vaccinated and unvaccinated mothers. This study is an important addition to a growing body of evidence surrounding the efficacy of COVID-19 vaccinations, especially in the population of pregnant patients.
Click to read the study in BMJ
Image: PD
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