Nirsevimab, a long-acting monoclonal antibody, has been found to be highly effective in preventing respiratory syncytial virus (RSV)-associated hospitalization among infants in their first RSV season, according to a recent report by the New Vaccine Surveillance Network. RSV is the leading cause of hospitalization among infants in the United States, with 50,000 to 80,000 hospitalizations occurring annually in children under the age of five. The highest rates of hospitalization occur during the first few months of life, and the risk decreases as infants get older.
In August 2023, the CDC’s Advisory Committee on Immunization Practices recommended nirsevimab for infants under eight months old to protect against RSV-associated lower respiratory tract infection during their first RSV season. The committee also recommended nirsevimab for children aged 8-19 months who are at increased risk for severe RSV disease. The recommendation came after phase 3 clinical trials showed that nirsevimab was 81% effective against RSV-associated lower respiratory tract infection with hospitalization through 150 days after injection.
The recent analysis by the New Vaccine Surveillance Network evaluated the effectiveness of nirsevimab against RSV-associated hospitalization among infants in their first RSV season from October 2023 to February 2024. Among the 699 infants hospitalized with acute respiratory illness, 59 received nirsevimab at least seven days before symptom onset. The analysis found that nirsevimab was 90% effective against RSV-associated hospitalization, with a median time from receipt to symptom onset of 45 days.
It is important to note that the number of infants who received nirsevimab in this analysis was relatively low, and the effectiveness of the antibody is expected to decrease over time due to antibody decay. However, this early estimate supports the current recommendation for the use of nirsevimab in preventing severe RSV disease in infants. The CDC advises that infants should be protected either by maternal RSV vaccination or by receiving nirsevimab.
The study has some limitations, including the limited availability of nirsevimab during the first season of introduction and intermittent supply shortages, as well as the fact that infants who received nirsevimab were more likely to have underlying medical conditions. Therefore, the results may not be fully generalizable to all eligible infants in their first RSV season.
In conclusion, the early estimate of nirsevimab effectiveness for the prevention of RSV-associated hospitalization among infants is promising. The findings support the current recommendations for the use of nirsevimab to reduce the risk of severe RSV disease in infants. Further studies are needed to assess the effectiveness of nirsevimab under real-world conditions for the full duration of an RSV season and in children aged 8-19 months at high risk for severe RSV disease.
