Real-World Effectiveness of RSV Vaccine in Pregnant Individuals

A newly approved and recommended nonadjuvanted bivalent respiratory syncytial virus (RSV) prefusion F (RSVpreF) protein subunit vaccine (Pfizer) is indicated for pregnant individuals between 32 0/7 to 36 6/7 weeks’ gestation during the 2023 to 2024 RSV season, however, there is a lack of clinical vaccine data available. In this cohort study of 2973 patients who delivered between 2023 and 2024 during the recommended vaccination period, 34.5% received prenatal RSVpreF vaccination. Maternal vaccination status did not show an increased risk of preterm birth (PTB).¹

Among 2973 pregnant individuals, 1026 received prenatal RSVpreF vaccination. There were 15 patients who incorrectly received the vaccine at 37 weeks gestation or later, who were grouped with the nonvaccinated cohort as a result. Of the 60 patients with evidence of prenatal vaccination during the study period, 5.9% experienced PTB compared to 131 among those who were not vaccinated (6.7%). Adjusting for potential confounders, prenatal vaccination did not show an increased risk for PTB (adjusted OR, .87; 95% CI, .62-1.20) and accounted for immortal time bias (HR, .93; 95% CI, .64-1.34). Logistic regression models did not reveal significant differences in pregnancy and neonatal outcomes based on vaccination status. Although, a heightened risk of hypertensive disorders of pregnancy (HDP) was observed in the time-dependent model (HR, 1.43; 95% CI, 1.16-1.77).

“The vaccine uptake among our patients is higher than what has been reported nationally. In contrast to our vaccination frequency of 34.5%, the CDC reports the overall coverage with the RSV vaccine nationally was 17.8% during the same time period,” according to the investigators, led by Moeun Son, MD, an assistant professor of Obstetrics and Gynecology at Weill Cornell Medical College.¹

According to the CDC, “As of January 31, 2024, the CDC reported that 17.8% of pregnant people who were at least 32 weeks gestation and had received the RSV vaccine before January 31, 2024, were covered.² Coverage rates varied by race and ethnicity, with non-Hispanic Asian pregnant people having the highest rate at 24.8% and non-Hispanic Black pregnant people having the lowest rate at 10.3%.”²

The CDC advises pregnant individuals to receive the RSV vaccine during the September to January RSV season across most of the continental US. Antibodies provided by the immunization pass to the baby, shielding them from severe RSV disease at birth. During the first 6 months postbirth, the vaccine can lower the baby's chance of RSV-related hospitalization by 57%.²

“Our study similarly found significant differences in the distribution of self-identified race and ethnicity based on vaccination status. We found a significantly lower vaccination frequency among patients who self-identified as Black or Hispanic and those with government insurance,” according to Son et al.¹

The primary outcome was PTB, defined as birth before 37 weeks gestation. Secondary outcomes included HDP, stillbirth, small-for-gestational-age birth weight, admission to the neonatal intensive care unit (NICU), neonatal respiratory distress requiring NICU admission, neonatal jaundice or hyperbilirubinemia, neonatal hypoglycemia, and neonatal sepsis. The analysis involved logistic regression models to calculate odds ratios (ORs), and both multivariable logistic regression and time-dependent covariate Cox regression models were utilized.

The strengths, according to investigators, included “provid[ing] clinical data for the first RSV season in the continental US during the postmarket period of an FDA-approved and nationally recommended RSV vaccine specific to the pregnant population. We had a higher proportion of vaccinated patients compared with nationally reported estimates. We included most patients who delivered at our 2 hospitals, which expands on the trial population, which included only patients with low risk. We explored important neonatal outcomes not previously reported in the trial. Fifth, we utilized several statistical approaches to minimize potential biases that commonly affect postmarket observational vaccine studies.”¹

The study's limitations include the NYC-based patient cohort as specific demographics may not generalize to broader populations; a reliance on EHR documentation for RSVpreF vaccination status, possibly missing vaccinations from smaller, unaffiliated sites, and leading to misclassification bias; a residual immortal time bias affecting PTB outcomes due to vaccination timing near full term; a reliance on ICD-10-CM codes for certain outcomes without individual confirmation; and a higher vaccination rate than national estimates, potentially impacting conclusions on PTB risk due to sample size limitations.

Overall, despite the study's limitations, it showed no increased risk of preterm birth associated with prenatal vaccination. The findings underscore disparities in vaccination rates across demographic groups and contribute valuable insights into vaccine safety and effectiveness during pregnancy, informing ongoing efforts to improve maternal and neonatal health outcomes.

References

1. Son M, Riley LE, Staniczenko AP, et al. Nonadjuvanted Bivalent Respiratory Syncytial Virus Vaccination and Perinatal Outcomes. JAMA Netw Open. 2024;7(7):e2419268. doi:10.1001/jamanetworkopen.2024.19268

2. CDC. For Immunization Managers. RSVVaxView. Published May 22, 2024. Accessed July 8, 2024. https://www.cdc.gov/vaccines/imz-managers/coverage/rsvvaxview/index.html