Immunization with the long-acting monoclonal antibody nirsevimab may provide stronger protection against hospitalizations for respiratory syncytial virus (RSV) in infants than maternal RSV vaccination during pregnancy, though the difference appears to disappear when maternal vaccination occurs at least eight weeks before delivery.
That’s according to a large French study published last week in The Lancet Child & Adolescent Health, in which a team led by researchers at Universite Paris Cite and Universite Sorbonne Paris Nord compared infant immunization with nirsevimab shortly after birth with maternal vaccination with the RSV prefusion F (RSVpreF, Abrysvo) vaccine during pregnancy.
22% lower odds of hospital stay with nirsevimab
For the population-based retrospective cohort study, the team analyzed health data from 164,140 infants born from September 2024 through February 2025. Of those newborns, 103,062 received nirsevimab after birth, and 61,078 were born to mothers vaccinated during pregnancy with RSVpreF vaccine. After matching infants by birth date, sex, and region, the researchers compared outcomes in 42,098 infants in each group.
At the six-month follow-up, 350 infants (0.83%) in the nirsevimab group were hospitalized for RSV-related lower respiratory tract infection (LRTI), compared with 403 infants (0.96%) in the maternal vaccine group. That translated to 22% lower odds of RSV-related hospitalization for the infants in the nirsevimab group (odds ratio [OR], 0.78).
We found that the longer the interval between maternal RSVpreF vaccination and delivery, the smaller the difference between the two products.
But the timing of maternal vaccination appeared to matter. When RSVpreF vaccination occurred at least eight weeks before delivery, researchers found no statistically significant difference between the two approaches (OR, 1.01). Similarly, no significant difference was seen when maternal vaccination occurred between 28 and 31 weeks of gestation.
“We found that the longer the interval between maternal RSVpreF vaccination and delivery, the smaller the difference between the two products,” write the researchers. “Overall, these findings highlight that the comparative effectiveness of nirsevimab at birth versus maternal RSVpreF vaccine is strongly influenced by the timing of maternal vaccination during pregnancy and suggest that early maternal vaccination during pregnancy could optimise its effectiveness.”
RSV a leading cause of childhood illness, death
RSV is the leading cause of LRTIs in children 5 years and younger and remains a major cause of illness and death worldwide. Infants in the first six months of life face the greatest risk, accounting for 1.4 million hospitalizations and 45,000 deaths annually.
The study had some limitations. The research was done during a single RSV season in France, so the findings may not be generalizable to other seasons with different circulating strains. The researchers also used medical billing and diagnostic codes to identify RSV-related infections, which means some cases could have been categorized incorrectly.
Still, say the authors, the findings add to the body of evidence suggesting that infant immunization with nirsevimab may be slightly superior to maternal RSVpreF vaccination in protecting newborns against RSV.
The data also suggest that, when maternal vaccination happens at least eight weeks before delivery, it may be similarly effective in reducing hospitalizations. “These results could help policy makers optimise public health decisions to maximise RSV prevention in infants,” the authors conclude.
