Can a RSV vaccine administered during pregnancy be effective in preventing infection during infancy?

Giving vaccines to mothers during pregnancy to protect the offspring following birth has been standard care for many years. Respiratory syncytial virus (RSV) is a common case of severe lower respiratory tract infection in infants, especially in younger infants. An investigation in the New England Journal of Medicine looked at whether giving expectant mothers a vaccine for RSV would have benefits for the newborn.¹

The investigators enrolled healthy pregnant women, who were at 28 weeks 0 days through 36 weeks 0 days of gestation, and had a delivery date that was supposed to occur near the start of the RSV season. The women were assigned to receive either a placebo or a single intramuscular dose of RSV fusion (F) protein nanoparticle, at a ratio of roughly 2:1. The offspring were followed for 180 days to evaluate outcomes linked to lower respiratory tract infection and 364 days to assess safety

Randomization occurred in 4636 women and there were 4579 live births. In the first 90 days of life, the percentage of infants who had a RSV-associated, medically significant lower respiratory tract infection was 1.5% in the vaccine group and 2.4% in the placebo group. Corresponding percentages for RSV linked lower respiratory tract infection with severe hypoxemia was 0.5% in the vaccine group and 1.0% in the placebo group. Additionally, hospitalizations for RSV-associated infections were 2.1% in the vaccine group and 3.7% in the placebo group. Adverse events among the pregnant women were similar between the vaccine and placebo group, except for local injection-site reactions, which were more common in the women who received the vaccine than the placebo (40.7% vs 9.9%).

The researchers said that there were multiple limitations to the study. One was that the study was underpowered because there had been an overestimation of the overall percentage of infants who had a primary end-point event, as well as the early end of the trial. Testing of the infants’ cord blood for RSV A and B neutralizing antibodies hasn’t been completed yet and those results are needed to clarify associations.

They concluded that administering a RSV F protein nanoparticle vaccine to pregnant women did not meet the success criterion for efficacy against RSV-linked and medically significant lower respiratory tract infection in infants during the first 90 days. They also said that further study would be needed to further elucidate their findings.

Reference

1. Madhi S, Polack F, Piedra P, et al. Respiratory syncytial virus vaccination during pregnancy and effects in infants. NEJM. 2020.383(5):426-439. doi:10.1056/nejmoa1908380