Immunization against respiratory syncytial virus delivered to infants following birth is safe and provides high antibody levels, regardless of the mother's RSV vaccination status, according to new research presented at IDWeek 2025. The findings address a critical knowledge gap about the safety of combining maternal vaccination with post-birth infant immunization.
Study Design and Methodology
The study examined 181 mother-infant pairs from eight sites across the United States, dividing them into four groups with different methods and timing of RSV immunization. The approaches included maternal vaccination with the RSVpreF vaccine during pregnancy only; maternal vaccination combined with infant nirsevimab immunization immediately post-birth; maternal vaccination with infant immunization three months after birth; and infant immunization immediately post-birth only.
Mothers and infants were assessed at birth and followed through four months of age, with immunity measured by RSV antibody levels.
Safety and Efficacy Results
The study's interim findings revealed that maternal vaccination and infant immunization, both alone and in combination, had no adverse effects on mothers or infants. All four groups demonstrated high antibody levels at three months, confirming the effectiveness of each approach.
"Our research reassures new parents that all methods of immunization for RSV are safe and protective, which is especially important as the United States moves into its wave of seasonal respiratory illnesses," said Christina A. Rostad, MD, director of the Emory Children's Center Vaccine Research Clinic and presenting author.
The research showed particularly dramatic results for infants whose mothers weren't vaccinated during pregnancy. These babies experienced a 25-fold increase in RSV protection after receiving the nirsevimab antibody shot, compared to a more than three-fold increase among those whose mothers were also vaccinated.
Clinical Significance
RSV represents the most common cause of hospitalization in children under one year and is the leading cause of lower respiratory tract infections, which can lead to complications like bronchiolitis and pneumonia. The Centers for Disease Control and Prevention recommends RSV vaccination for people between 32-36 weeks pregnant as well as immunization for all infants under eight months not otherwise protected through vaccination during pregnancy.
While most infants do not need both products to provide protection through their first respiratory season, there are clinical scenarios when infants may receive both. This study provides additional information on the safety and immunity provided by dual product administration.
Current Implementation Challenges
Previous research published in Pediatrics found that about 43% of babies received a nirsevimab injection during the 2023-24 cold and flu season, the first season following FDA approval. However, more than two-thirds of mothers (68%) did not receive an RSV vaccine during pregnancy, and overall, about 28% of infants went without any immunization against RSV.
Ongoing Research
The study continues as researchers will follow participants to assess immune durability for one year in mothers, infants, and breast milk. While the vaccine is proven safe and well-tolerated in infants, researchers note that durability of immunity for mothers and infants has yet to be fully described.
The research is being conducted by the Infectious Diseases Clinical Research Consortium through funding from the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health.
