An interim analysis of a phase IIb trial indicated that a bivalent protein-based RSV vaccination given during pregnancy was safe and resulted in stronger neutralizing antibody responses in women and also protected their infants.

A total of 406 women and 403 newborns participated in the study, among whom 327 received RSV prefusion F protein-based (RSVpreF) vaccination. Participants were assigned to one of two vaccination doses (120 g or 240 g) made with or without aluminium hydroxide, or a placebo.

A higher geometric mean ratio of 50% neutralizing antibody titers against RSV A and B was seen in the maternal serum and umbilical cord blood compared to the placebo group. The vaccine showed 84.7% (95% CI 21.6-97.6) effectiveness and 91.5% (95% CI -5.6 to 99.8) effectiveness against medically attended RSV-associated and severe RSV-associated lower respiratory tract illness

Nonaluminum formulations had higher transplacental neutralizing antibody transfer ratios than aluminium formulations, ranging from 1.41 to 2.10. Infants of vaccinated mothers showed equivalent titers in umbilical-cord blood and similar transplacental transfer ratios across the range of gestational ages studied. Mild or moderate post-vaccine adverse events (AEs) were reported, and no serious AEs. 

Thus the RSVpreF vaccine induced neutralizing antibody responses with effective transplacental transfer and no obvious side effects. (MedPage Today April 27, 2022),N Engl J Med 2022; DOI: 10.1056/NEJMoa2106062.