Boosting maternal and neonatal anti SARS-CoV-2 humoral immunity using a third mRNA vaccine dose

Abstract Importance: To minimize COVID-19 pandemic burden and spread, third booster dose vaccination campaigns commenced worldwide. Since pregnant patients are at increased risk for severe disease, they were recently included in that policy despite the lack of available evidence regarding the impact of a third boosting dose during pregnancy; underscoring the urgent need for relevant data. Objective: We aimed to characterize the effect on anti-SARS-CoV-2 antibody titers of the third, boosting dose of mRNA Pfizer BNT162b2 vaccine in pregnancy, and profile its most common side effects. Design: Prospective cohort study of anti-SARS-CoV-2 antibody titers measured at the time of delivery in maternal and cord blood, and dedicated side effect questionnaire. Setting: Labor and delivery ward and virology laboratory of a large, urban, university-affiliated medical center.Participants: Gravidae (N=216) without history of COVID-19 disease, presenting for delivery were divided to two groups by vaccination status: parturients who received a third boosting dose of Pfizer BNT162b2 mRNA vaccine, were compared to those vaccinated with the 2-dose regimen. Main Outcomes: Anti-SARS-CoV-2 antibody titers measured in maternal and cord blood samples collected at delivery and side effect profile.Results: We found a robust surge in maternal and cord blood levels of anti-SARS-CoV-2 titers at the time of delivery: parturients that received a third boosting dose had titers 4.7-fold higher as compared to 2-dose vaccinated parturients. The effect of the third boosting dose remained significant when controlling for the trimester of most recent vaccine dose, suggesting additive immunity extends beyond that obtained after the second dose. Efficient transplacental transfer was observed. Neonatal (cord blood) anti-SARS-CoV-2 antibody titers positively correlated with (r= 0.745; P< .0001) and were significantly higher than maternal levels (p<0.0001). Overall, fewer side effects were reported following the third dose as compared to the second vaccine dose among the 2-dose group, with lower rates of injection site pain and swelling, myalgia, and general malaise. Conclusions and relevance: The third, boosting dose of mRNA Pfizer BNT162b2 vaccine augmented maternal and neonatal immunity with mild side effects. These data provide essential evidence to bolster clinical and public health guidance, reassure patients, and increase vaccine uptake among pregnant patients.