Maternal influenza vaccination preferentially boosts hemagglutinin stem‐specific antibody resulting in efficient transplacental transfer of stem‐specific IgG.

Objective: The objective of this study was to evaluate hemagglutinin stem‐specific antibody response to the influenza vaccine during pregnancy and its transfer to the infant. Methods: The authors assessed antibody titers among maternal participants and their paired neonate's cord blood (CB) using enzyme‐linked immunoassay. Fifteen pregnant participants pre‐2019 and post‐2019 seasonal influenza vaccine were compared with 18 prenatally vaccinated participants with paired neonatal CB samples. Total IgG and IgG subclass titers specific for whole vaccine antigens versus recombinant hemagglutinin stem–specific antigen were compared using Wilcoxon exact test. Results: Hemagglutinin stem–specific IgG was boosted more robustly than whole vaccine titers when comparing postvaccine versus prevaccine log2 IgG ratios (P = 0.04). Hemagglutinin stem–specific IgG titers were boosted postvaccination (prevaccine: 14.5 [95% confidence interval, 13.8–15.2] vs. postvaccine: 16 [95% confidence interval, 15.2–16.8], P = 0.004). While IgG to whole vaccine was similar in neonatal CB and maternal plasma (P = 0.09), hemagglutinin stem–specific IgG concentrated in CB (P = 0.002), which was dominated by IgG1 subclass (analysis of variance P < 0.05). Conclusion: These data demonstrate the ability of pregnant women to generate a more robust antibody response to the stem region compared with the head region of hemagglutinin with transplacental transfer of IgG. Synopsis: Influenza vaccination during pregnancy elicits robust anti‐hemagglutinin stem antibody response, which is IgG1 dominant and transfers efficiently to the fetus. [ABSTRACT FROM AUTHOR]