Nirsevimab Reduces RSV Risk in Young Children, But Low Vaccine Uptake Limits Impact

A new study published in JAMA Pediatrics highlights the ongoing burden of Respiratory Syncytial Virus (RSV) in young children and evaluates the effectiveness of nirsevimab, a newly recommended RSV prevention product, during the 2023-2024 RSV season in the United States. The study found that RSV continues to be a significant health issue for children under five years old, with hospitalization rates during the 2023-2024 season similar to those seen in previous seasons (2017-2020). The hospitalization rate for RSV among children under five was five per 1,000, with the highest rates observed in infants aged zero to two months (26.6 per 1,000).¹

The study also assessed nirsevimab’s effectiveness, finding that it significantly reduced the risk of medically attended RSV-associated acute respiratory illness (ARI). Among infants who received nirsevimab, the vaccine was 89% effective in preventing RSV-associated ARI (95% CI, 79%-94%) and 93% effective (95% CI, 82%-97%) in preventing RSV-associated hospitalization. Although, uptake of the vaccine remained low, with only 1% of RSV-positive children and 15% of RSV-negative controls receiving nirsevimab during the study period. Additionally, researchers examined RSV specimens for potential mutations in the nirsevimab binding site but found no differences between infants who received the vaccine and those who did not.¹

The investigators noted that "Our analysis suggests that RSV circulation may be returning to prepandemic patterns with respect to timing, the distribution of RSV-A and RSV-B subtypes, and incidence rates of hospitalization by age. Predictable seasonality will facilitate RSV prevention product implementation and effectiveness assessment efforts that depend on anticipating season onset."¹

This case-control study, which included 28,689 children under 5 years old with medically attended ARI, compared the RSV burden and disease epidemiology in the 2023-2024 season with data from three prepandemic RSV seasons (2017-2020). The findings suggest that while RSV remains a significant concern, nirsevimab has the potential to reduce the disease burden, particularly if vaccine coverage increases in the future.¹

Several limitations should be considered when interpreting these findings. First, the study was conducted at academic pediatric health systems, which may not be nationally representative, although RSV incidence and severity in this analysis align with other US studies. Second, nirsevimab was available at most sites only after RSV circulation began, resulting in a shorter interval between administration and illness onset. This limited window may have led to a lower estimate of nirsevimab’s effectiveness over an entire RSV season, due to antibody decay.¹

Similarly, a recent study published in Pediatrics found that nirsevimab was 76% effective in preventing medically attended RSV-related lower respiratory tract infections (LRTIs) in infants in primary care settings in Spain. The study also showed that catch-up immunization programs, where nirsevimab was administered outside hospital settings before the RSV season, yielded even higher efficacy (80.2%).² Both studies emphasize the effectiveness of nirsevimab in reducing RSV-related illness, with the Spanish study focusing on community-based prevention and the US study on hospital prevention. Together, these findings reinforce the broader potential of nirsevimab across different healthcare settings.

In conclusion, this study underscores the potential for a substantial public health impact with increased and equitable coverage of RSV prevention products in future RSV seasons. While the 2023-2024 RSV season demonstrated that nirsevimab is effective in preventing severe disease, broader distribution and higher vaccination rates will be essential to reducing the burden of RSV infections among young children.

References

1. Moline HL, Toepfer AP, Tannis A, et al. Respiratory Syncytial Virus Disease Burden and Nirsevimab Effectiveness in Young Children From 2023-2024. JAMA Pediatr. Published online December 09, 2024. doi:10.1001/jamapediatrics.2024.5572

2. López-Lacort M, Muñoz-Quiles C, Mira-Iglesias A, et al. Nirsevimab effectiveness against severe respiratory syncytial virus infection in the primary care setting. Pediatrics. 2024; e2024066393. doi:10.1542/peds.2024-066393