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Prenatal RSV Vaccine Not Associated with Preterm Birth, According to First Real-World Data


Data from a study performed during the first season the shot was available expand on clinical trial evidence and support the safety of RSVpreF, authors wrote.

Women who received prenatal vaccination with the bivalent RSV prefusion F (RSVpreF) vaccine (Abyrsvo; Pfizer) were not at increased risk of preterm birth or other adverse outcomes, according to new research from Weill Cornell Medicine and New York-Presbyterian researchers.1

image credit: nursing mother: ©yepifanovahelen/stock.adobe.com

Findings of the retrospective cohort study were published today in JAMA Network Open and highlighted the association between prenatal RSV vaccination and perinatal outcomes among women who delivered during the 2023-2024 RSV season, the first during which the vaccine was available.

Based on phase 3 clinical trial data,2 the US FDA approved the nonadjuvanted bivalent RSVpreF vaccine in August of 2023 for pregnant individuals between 32 and 36 weeks of gestation. In September, the CDC’s Advisory Committee on Immunization Practices (ACIP) recommended that the vaccine be administered to all eligible pregnant women during the RSV season, September to the end of January. Until the present investigation, there has been no real-world clinical evidence on the safety of the shot.

“The real-world evidence provides an additional layer of confidence about the safety of this vaccine during pregnancy,” lead author Moeun Son, MD, associate professor of obstetrics and gynecology at Weill Cornell Medicine, said in a press release.3 “Randomized clinical trials don’t always emulate the populations we see in the clinical setting, but now we have data from multiple populations showing no increase in preterm birth risk.”3

Son and colleagues conducted their research at 2 metropolitan New York hospitals, enrolling women with singleton births at 32 weeks’ gestation or later from September 22, 2023, to January 31, 2024. Receipt of prenatal RSV vaccination was captured from the health system’s electronic health records, according to the study.1

In addition to the preterm birth rate (less than 37 weeks’ gestation), Son et al also recorded hypertensive disorders of pregnancy, still birth, small for gestational age birth weight, neonatal intensive care unit admission, and other adverse outcomes.


A total of 2973 women (median age 34.9 years) were included. More than half (56.7%) identified as White, 20.8% as Asian, 6.5% as Black or African American, 9.9% as Hispanic, Latina, or of Spanish origin, and 8.3% as other race/ethnicity. Among them, 1026 (34.5%) received the RSVpreF vaccine and 1947 (65.5%) did not. Mean gestational age, according to the study, was 34.5 weeks.

During the study period 60 (5.9%) of the participants who had received the vaccination experienced preterm birth compared with 131 (6.7%) of those who had not been vaccinated, evidence that RSVpreF vaccination during pregnancy was not associated with increased risk for the outcome (OR, 0.88; 95% CI, 0.64-1.20). The researchers found no difference after multivariable adjustment (aOR,0.87; 95% CI, 0.62-1.20).

In the time dependent model, the researchers reported, they did find a significantly increased risk of overall hypertensive disorders of pregnancy in the vaccinated group (HR, 1.43; 95% CI, 1.16-1.77), a finding also observed in the phase 3 registration trial for the RSVpreF vaccine although the associations in that study did not reach statistical significance, authors wrote. In stratified analyses, Son et al found the significant increased risk appeared to be associated with hospital site and insurance type, findings they feel should be investigated in other populations and settings.

In their overall analyses, Son and colleagues found rates of secondary outcomes were similar between the vaccinated and unvaccinated groups, including for stillbirths, small-for-gestational-age birthweight, neonatal intensive care unit (NICU) admissions, respiratory distress with NICU admissions, jaundice, hypoglycemia, and sepsis among the newborns, according to the study.

Comparative data

Vaccine frequency in the cohort during the study period was 34.5%,1 nearly 2 times greater than the overall coverage for the nation of 17.8% reported by the CDC for the same interval.4 The researchers suggest efforts at the hospitals involved in the study to promote vaccination, including stocking and administering the shot at most of the clinical sites, contributed to the significant gap in uptake.

Data from the same CDC report showed that vaccination with RSVpreF was highest among non-Hispanic Asian (24.8%) and lowest among non-Hispanic Black individuals.4 Vaccination rates in their study were similarly lower among participants who self-identified as Black or Hispanic and among those with government insurance, Son et al reported. The authors called for further investigation of barriers to RSVpreF vaccination, including general vaccine hesitancy, poor access, cost, and preference for use of nirsevimab for the neonate. (Nirsevimab was in very limited supply during the study period.)1 

Among the study’s limitations the authors noted the population attending the New York-based study hospitals may limit the ability to generalize their findings. The use of EHR data for vaccination status may have missed shots given in non-clinical settings, introducing potential bias toward the null, they added, and the sample size may have been underpowered, with the risk of potential type 2 errors.1

Despite the drawbacks, however, the findings provide clinical data for the first US RSV season during the postmarketing period of the RSVpreF vaccine, expand on data from the registration trial population, and shed light on important neonatal outcomes not explored in that trial.1


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. Kampman B, Madhi SA, Junjal I, et al; for the MATISSE Study Group. N Engl J Med. 2023;388:1451-1464
3. Study backs RSV vaccine safety during pregnancy. News release. Weill Cornell Medicine. July 8, 2024. Accessed July 8, 2024. https://news.weill.cornell.edu/news/2024/07/study-backs-rsv-vaccine-safety-during-pregnancy
4. US Centers for Disease Control and Prevention. RSVVaxView. Last reviewed June 21, 2024. Accessed July 9, 2024. https://www.cdc.gov/vaccines/imz-managers/coverage/rsvvaxview/index.html

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