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Alisa Kachikis, MD, MSc, assistant professor of obstetrics and gynecology at the University of Washington in Seattle, provided an update on maternal RSV immunizations at IDWeek 2023.

Globally, she noted, respiratory syncytial virus (RSV) is associated with 33 million lower-respiratory tract infections annually and leads to death in 1 in 28 affected infants <6 months. The greatest concentration of disease burden is in low to middle income countries. In pregnancy, RSV is thought to be associated with ~10% of lower respiratory tract infections in pregnant individuals. Pregnant patients with underlying pulmonary conditions are at greatest risk of complications.

Maternal immunization can prevent disease in infants: Maternal IgG is actively transferred across the syncytiotrophoblast cells in placental chorionic villi during pregnancy, starting in the second trimester, and with more efficient transfer in the third trimester.

Kachikis also noted that maternal vaccine development and implementation must consider the safety for the pregnant person and the fetus and factors that affect IgG transfer, including timing of the vaccine, gestational age at delivery, maternal antibody concentrations, the nature of the antibody, and maternal comorbidities.

Kachikis shared information on an international phase 3 double blind placebo-controlled trial recently published in The New England Journal of Medicine, which examined the efficacy and safety of administering a bivalent RSV pre-fusion F protein-based vaccine to pregnant recipients to prevent infants against RSV associated illness in early life2. In the trial, 7392 pregnant participants at gestational ages 24 through 36 weeks were randomized to receive a single 120 mcg dose of vaccine vs placebo. Vaccine efficacy for medically attended severe RSV-associated LRTI was 81.8% at 90 days and 69.4% at 180 days of life.

The most commonly reported adverse effects in vaccine recipients were pain at the injection site, headache, muscle pain, and nausea. The adverse events, serious adverse events, and severe adverse events in vaccine recipients were similar in number to placebo recipients. No safety concerns were identified in infants.

However, a non-statistically significant numerical imbalance in preterm births was noted in vaccine recipients (5.7%) vs placebo recipients (4.7%). The numerical imbalance was not seen in data from participants from the U.S. or other high income countries, and was noted primarily from data from participants in a single middle-income country.

In August 2023 the FDA approved the RSV vaccine Abrysvo for use in pregnant individuals to prevent LRTI caused by RSV in infants <6 months. To avoid any potential risk of preterm birth with use of Abrysvo before 32 weeks of gestation, the vaccine is approved for use in pregnant individuals at 32 through 36 6/7 weeks gestational age. The Advisory Committee on Immunization Practices, the American Congress of Obstetricians and Gynecologists, and the Society for Maternal-Fetal Medicine all recommend a single dose of the RSV vaccine to be administered to pregnant individuals between 32 and 36 6/7 weeks gestation, while the European Medicines Agency has approved use of the vaccine between 24 and 36 weeks.

Kachikis expresses enthusiasm for the vaccine, with some disappointment at the restrictive gestational age administration recommendation from the US societies. Post-marketing surveillance will be essential for monitoring adverse pregnancy outcomes. If additional data confirms the safety of the vaccine with regard to preterm births then extending the gestational age administration timeframe could potentially expand the protection to more infants including those born preterm. The ideal timeframe of administration with regard to seasonality is an additional question that needs to be further studied. Kachikis also emphasizes the need for promoting health equity and ensuring access in geographic regions with the greatest burden of RSV-associated morbidity and mortality.

Reference
Kachikis A. Maternal RSV Vaccine. October 14, 2023. IDWeek 2023. Boston, Massachusetts.