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Administration of the maternal respiratory syncytial virus prefusion F (RSVpreF) vaccine (Abrysvo) didn't come with an increased risk of preterm birth or other negative neonatal outcomes, according to a retrospective study.

Preterm birth, before 37 weeks' gestation, was actually numerically less common for women who received Pfizer's maternal RSV vaccine than without it (5.9% vs 6.7%, adjusted OR 0.87, 95% CI 0.62-1.20), Moeun Son, MD, of Weill Cornell Medical College in New York City, and colleagues reported in .

No significant differences emerged by vaccination status in small-for-gestational-age birthweight, stillbirth, neonatal ICU admissions, respiratory distress with NICU admissions, or neonatal jaundice, hypoglycemia, or sepsis. Percentages of spontaneous and nonspontaneous premature births were similar as well.

"Patients and clinicians can feel confident that the vaccination during pregnancy is a safe way to protect infants from harmful RSV infections," Son told 木瓜直播 in an email.

"The data are particularly reassuring," she added, "because we include a broader group of women, including some with health conditions that could increase the risks of poor pregnancy outcomes who were excluded" from the MATISSE study, the phase III clinical trial that led to the vaccine's approval.

However, in a time-dependent model, researchers did find an association of the vaccine with hypertensive disorders of pregnancy. Among women who received the vaccine, 20.1% were diagnosed with a hypertensive disorder during pregnancy versus 18.1% of the group that received no vaccine (HR 1.43, 95% CI 1.16-1.77).

This initial postmarketing study suggests "the benefits of the vaccine greatly outweigh the risks," Annette Regan, PhD, MPH, of the University of San Francisco's School of Nursing and Health Professions, wrote in .

"While these data provide initial reassurance that the vaccine was not associated with preterm birth in this setting, there are important limitations to consider, and some findings warrant further investigation," Regan cautioned. "Results were not stratified by onset of labor, making it difficult to disentangle whether preterm birth with a spontaneous onset may differ by vaccination status," she noted.

Before Pfizer's vaccine received FDA approval in August 2023, phase III clinical trial data raised concerns about a observed in women who had received the vaccine compared with women in the control group. Women in those trials received the vaccine from weeks 24 through 36 of pregnancy. Given this potential risk, the FDA limited approval for Pfizer's vaccine to use during gestational .

Of note, GSK recently decided to halt a phase III clinical trial of a similar RSVpreF vaccine because of a 1.9% absolute higher rate of preterm birth observed in the intervention arm. In that trial, women randomized to the vaccine arm received it between 24 and 34 weeks of gestation.

Regan also speculated that the real-world study drew from a population that is healthier than the general U.S. birthing population, given that the 6-7% preterm birth rate observed in this cohort is lower than the national preterm birth rate of 10-11%. "Replication in larger and more diverse cohorts of US pregnancies would therefore be helpful," she wrote.

The retrospective observational cohort study looked at data from 2,973 pregnant women from two New York City hospitals within one healthcare system who gave birth at 32 weeks of gestation or later from September 2023 through January 2024. Of these, 34.5% had received the prenatal RSV vaccine. The median age was about 35 years.

The maternal RSV vaccination rate at the hospitals was almost double that of the nationally reported rate of 17.8% during the study period, Son noted.

Researchers found some disparities in vaccination rates, she said. Black women, women with public insurance, and those who had fewer prenatal visits were less likely to be vaccinated. These disparities are similar to those previously seen for other recommended vaccines during pregnancy, she pointed out.

"As we prepare for the next RSV vaccination season, which starts this September, we are going to speak with different communities to help understand what might cause vaccine hesitancy among women or create barriers to access the vaccine, so we can improve vaccination rates even more this year," Son told 木瓜直播.

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