Early Estimate of Nirsevimab Effectiveness for Prevention of Respiratory Syncytial Virus–Associated Hospitalization Among Infants Entering Their First Respiratory Syncytial Virus Season

Nirsevimab, a long-acting monoclonal antibody, has been found to be highly effective in preventing hospitalization due to respiratory syncytial virus (RSV) in infants during 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 grow older.

In August 2023, the CDC’s Advisory Committee on Immunization Practices recommended nirsevimab for infants under the age of eight months to protect against RSV-associated lower respiratory tract infection during their first RSV season. The recommendation also included children aged eight to 19 months who are at an increased risk for severe RSV disease. The effectiveness of nirsevimab was previously evaluated in phase 3 clinical trials, where it was found to be 81% effective against RSV-associated lower respiratory tract infection with hospitalization within 150 days after administration.

The current analysis by the New Vaccine Surveillance Network aimed to assess the post-introduction effectiveness of nirsevimab in preventing RSV-associated hospitalization among infants during their first RSV season. The study included 699 infants hospitalized with acute respiratory illness between October 2023 and February 2024. Of these infants, 59 (8%) had received nirsevimab at least seven days before symptom onset. The effectiveness of nirsevimab against RSV-associated hospitalization was estimated to be 90% 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 is expected to decrease over time due to antibody decay. However, the early estimate of effectiveness supports the current recommendation for the use of nirsevimab in preventing severe RSV disease in infants during their first RSV season. The CDC advises that infants should be protected through either maternal RSV vaccination or infant receipt of nirsevimab.

The study has several limitations, including the limited number of infants who received nirsevimab and the inability to stratify the estimates by the duration from receipt of nirsevimab. Additionally, nirsevimab became available at most sites in the United States after the start of the RSV season, which might have affected the estimated effectiveness. Further studies are needed to assess the effectiveness of nirsevimab against outpatient and emergency department visits related to RSV.

In conclusion, the early estimate of nirsevimab effectiveness in preventing RSV-associated hospitalization among infants during their first RSV season is promising. The findings support the current recommendations for the use of nirsevimab or maternal RSV vaccination to reduce the risk of severe RSV disease in infants. As more data becomes available, the CDC will continue to monitor the effectiveness of nirsevimab in real-world conditions and in older children at high risk for severe RSV disease.