A study in Jama Network Open in July 2024 explores the possibility of association between nonadjuvanted bivalent respiratory syncytial virus prefusion F (RSVpreF) protein subunit vaccination during pregnancy and preterm birth. The researchers find no increased risk of preterm birth, adding to existing evidence in support of the safety of prenatal RSVpreF vaccination with Pfizer’s Abrysvo. The team at Weill Cornell Medicine hope that their results will bring confidence to pregnant people considering vaccination and healthcare providers.
RSV concerns
The authors state that respiratory syncytial virus (RSV) contributes to between 58,000 and 80,000 hospitalisations each year in the US and between 100 and 300 annual deaths in children younger than 5 years. Recent strategies for RSV infection prevention in infants have been approved by the US FDA:
- Seasonal administration of a nonadjuvanted bivalent recombination RSV prefusion F (RSVpreF) protein subunit vaccine (Pfizer) to pregnant individuals
- Postnatal nirsevimab (monoclonal antibody) for infants aged up to 8 months
However, during the 2023 to 2024 RSV season, limited supply of nirsevimab shifted importance to prenatal vaccination. In September 2023, the US CDC’s Advisory Committee on Immunisation Practices recommended the RSVpreF vaccine to be administered to most pregnant individuals from September to January in the continental US. This contrasts GSK’s RSV adjuvanted vaccine, which is not approved for use in pregnancy after a trial was terminated early due to an elevated risk of premature birth and associated neonatal deaths.
Although the RSVpreF vaccine was approved, the gestational age window was limited to 32 0/7 to 36 6/7 weeks in response to concerns about the numerical difference in preterm birth (PTB) among participants who received the RSVpreF vaccine from 24 to 36 weeks’ gestation. While the FDA recommendation has been largely endorsed by obstetric care professionals, clinical data from the US 2023 to 2024 RSV season are “lacking”. Thus, the authors wanted to examine the uptake of RSVpreF vaccination among a medically and demographically diverse pregnant population in that season and compare those who were prenatally vaccinated with those who were not.
Study findings
Between 22nd September 2023 and 31st January 2024, 2,973 eligible pregnant individuals were included. Among them, 1,026 had electronic health record (EHR) evidence of RSVpreF vaccination before delivery and 1,947 did not. The mean gestational age at time of vaccination was 34.5 weeks. Overall preterm birth (PTB) occurred in 191 of 2,973 patients (6.5%) in the cohort, and evidence of RSVpreF vaccination during pregnancy was “not significantly associated with an increased risk for PTB”. In the time-dependent model an increased risk of overall hypertensive disorders of pregnancy (HDP) was observed in the vaccinated group, as well as more cases of gestational hypertension and preeclampsia. However, these were “not statistically significant”.
Providing confidence
Dr Moeun Son, associate professor of obstetrics and gynaecology at Weill Cornell Medicine, hopes that the “real-world evidence provides an additional layer of confidence” about the safety of the vaccine during pregnancy”.
“Randomised 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. Patients and clinicians can feel confident that vaccination during pregnancy is a safe way to protect infants from harmful RSV infections.”
Further research is required to understand whether the three analyses conducted reveal genuine concerns or differences between the two groups; for example, vaccinated women were more likely to have insurance or to have undergone in vitro fertilisation.
“These are things we will continue to explore in future studies.”
Additionally, Dr Son’s team will be speaking to “different communities” to understand the causes of vaccine hesitancy and “barriers to access”.
“We want to ensure that all who would benefit will receive the vaccine.”
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