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Immunizing infants in their first respiratory syncytial virus (RSV) season with nirsevimab (Beyfortus) proved highly protective against severe disease, a case-control study showed.

From October 2023 through February 2024, the long-acting monoclonal antibody was 90% effective against RSV-associated hospitalization among a cohort of infants hospitalized in the U.S. for acute respiratory infection, according to researchers led by Heidi Moline, MD, of the CDC in Atlanta.

"This early effectiveness estimate supports existing recommendations for the prevention of severe RSV disease in infants in their first RSV season," the team wrote in .

However, the study timeframe only covered part of a typical RSV season, and Moline and co-authors cautioned that nirsevimab's effectiveness over a full RSV season would likely be lower, as antibody levels from passive immunization wane over time.

"In this analysis, the median interval from receipt of nirsevimab was 45 days, whereas the median duration of the U.S. RSV season before the COVID-19 pandemic was 189 days," they explained. "In clinical trials, nirsevimab remained highly efficacious against RSV-associated lower respiratory tract infection in infants through 150 days after receipt of nirsevimab, consistent with an extended half-life of 63-73 days."

RSV is the leading cause of hospitalization among U.S. infants, and each year leads to 50,000 to 80,000 hospitalizations for children under 5 years of age.

Last year, the CDC's Advisory Committee on Immunization Practices recommended either nirsevimab or the maternal RSV vaccine (Abrysvo) to protect infants born during or entering their first RSV season. Nirsevimab is also recommended for high-risk children ages 8 to 19 months entering their second RSV season.

But since its rollout, nirsevimab has been plagued by shortages, resulting in rationed supplies during the study period. CDC in October 2023 recommended that clinics prioritize the monoclonal antibody for the youngest infants and those with high-risk underlying conditions. Supply has stabilized, however, and the CDC in January recommended that clinics revert to the original ACIP recommendations.

The current analysis represents the first data on the real-world use of nirsevimab. To assess the immunization's effectiveness, Moline and colleagues examined infants hospitalized with acute respiratory infection at four U.S. sites (Pittsburgh, Seattle, Houston, and Nashville, Tennessee) in the New Vaccine Surveillance Network -- a population-based, prospective surveillance platform for acute respiratory infection in children.

Sites needed to have at least five infants enrolled who had received nirsevimab 7 or more days before symptom onset for inclusion. Infants were eligible if they were less than 8 months of age or born after the start of the study period (Oct. 1, 2023 to Feb. 29, 2024), had a verified nirsevimab status, and a medical record review for underlying medical conditions.

Of the 699 infants who met the study criteria, 407 tested positive for RSV (case patients) and 292 tested negative (controls). Most were 6 months of age or younger (83%), were born at term (79%), while 58% were boys and 94% had no high-risk medical condition.

Overall, just 1% of the patients testing positive for RSV had received nirsevimab, as compared with 18% of those testing negative. Receipt of the monoclonal antibody ranged from 4% to 12% by site, and was more frequent among infants with high-risk underlying medical conditions, 46% versus 6% for children without these conditions (P<0.001). There was no difference in the frequency of receipt of the monoclonal antibody by preterm status or insurance type.

Limitations of the analysis included that only a small proportion of hospitalized infants with an acute respiratory infection had received nirsevimab, likely due in part to its delayed availability and shortages, and that those who received it were more likely to have underlying conditions. Also, it's possible some infants in the analysis may have had RSV before receipt of nirsevimab, according to Moline and colleagues.

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