Feb. 19, 2025 – A vaccine for pregnant women that protects babies from respiratory syncytial virus (RSV) appears safe, according to two new studies that give researchers hope for widespread immunity against the disease, the leading cause of infant hospitalization in the United States.
Women given the RSV vaccine between weeks 32 and 36 of pregnancy showed no link between the vaccine and preterm birth, a concern raised by earlier trials.
One of the new studies “provides reassuring data” that the RSV vaccine is safe, said study co-author Christine Blauvelt, MD, a maternal-fetal medicine fellow at UCSF Health in San Francisco.
“We found no increased risk of pregnancy complications including no difference in preterm birth,” she said.
In the new study, 64% of pregnant people were vaccinated before giving birth, and 70% of newly born eligible babies got a monoclonal antibody treatment called nirsevimab. Combined, 80% of babies were protected for the 2023-24 RSV season.
This two-pronged approach led to “high uptake and reassuring perinatal outcomes,” the study concluded.
In some communities, the protection rate among babies is so high that it could provide population-level prevention of virus spread, according to the studies, published today in JAMA Network Open.
Severe complications from RSV lead to 80,000 annual hospitalizations among children under age 5. Worldwide, RSV accounts for 3.6% of infant deaths ages 28 days to 6 months.
When they were approved in 2023, the RSV immunizations faced a rise in vaccine hesitancy. In the study, mothers in California who declined the new vaccine often opted for the monoclonal antibody treatment, which technically is not a vaccine.
“We know that infant RSV infections can lead to serious complications including hospitalization, intubation, and chronic respiratory disease,” Blauvelt said. “Prenatal RSV vaccination and infant monoclonal antibodies against RSV are safe and effective ways to protect babies against RSV during the first 6 months of life when they are most vulnerable to this disease.”
The analysis included babies who were born at the UCSF Birth Center from October 2023 to April 2024 and whose mothers received prenatal care through UCSF. About 650 mothers were included in the analysis, and there were 260 babies whose moms didn’t get vaccinated and were eligible for the monoclonal antibody treatment.
The second study, also covering the 2023-24 RSV season, was done in Boston at a single medical center. The uptake rate was lower – just under 21% of pregnant people got vaccinated and 35% of babies got monoclonal antibodies. The study period was different and was likely impacted by nationwide supply issues early on, but the number of people in the study was larger – just under 2,000 pregnant people and newborns.
“Had we looked only at October 15 to January 1, my guess is our numbers would have been largely similar,” said researcher Ai-ris Yonekura Collier, MD, an assistant professor of obstetrics, gynecology, and reproductive biology at Harvard Medical School.
The strong combined uptake of the two options suggest that RSV herd immunity may be within reach, although a CDC report suggests lower uptake rates when looking at nationwide data.
“In locations in Spain where they achieved nearly 90% nirsevimab rates, there is substantial real-world effectiveness data for RSV prevention,” said Collier, who is also co-director of the Center for Virology and Vaccine Research Clinical Trials Unit at Beth Israel Deaconess Medical Center.
The studies also emphasized the importance of having the immunizations on hand and available, particularly at OB/GYN offices – 78% of the RSV vaccines in the California study were given during prenatal visits.
“Having the RSV vaccine available in prenatal clinics and nirsevimab available on labor and delivery units may reduce logistical barriers to vaccination,” Blauvelt said.
