Nirsevimab shows strong real-world effectiveness against RSV in infants

Nirsevimab shows strong real-world effectiveness against RSV in infants | Image credit: © Peter Hansen - stock.adobe.com.

A systematic review and meta-analysis published May 1, 2025, in The Lancet Child & Adolescent Health found that nirsevimab, a long-acting monoclonal antibody approved in 2023, is highly effective in preventing respiratory syncytial virus (RSV)-related outcomes in infants. The analysis of observational studies across 5 countries offers the first pooled assessment of nirsevimab’s effectiveness outside of clinical trials.^1,2

The study included 32 cohort and case-control studies from France, Italy, Luxembourg, Spain, and the United States. Of these, 27 studies were included in the meta-analysis, all of which compared infants who received nirsevimab with those who did not during the same RSV season (2023–2024). The primary analysis focused on infants aged 0–12 months, the population targeted by most infant immunisation programs.

The findings showed that nirsevimab was associated with an 83% reduction in the odds of RSV-related hospitalisation in infants under 12 months (odds ratio [OR] 0.17, 95% confidence interval [CI] 0.12–0.23). Across the included studies, 1585 hospitalisation events were recorded in the nirsevimab group (85,242 infants) compared with 3978 events in the control group (51,893 infants).

The analysis also found a significant reduction in RSV-related ICU admissions. Among 127,568 infants included in 9 studies, nirsevimab was associated with an 81% reduction in the odds of ICU admission due to RSV (OR 0.19, 95% CI 0.12–0.29).

In addition, the odds of developing RSV-related lower respiratory tract infection (LRTI), including conditions such as bronchiolitis and pneumonia, were lower in the nirsevimab group. Seven studies involving 57,872 infants found a 75% reduction in the odds of LRTI (OR 0.25, 95% CI 0.19–0.33).

However, the length of hospital stay did not differ between the 2 groups. Data from 6 studies involving 2606 infants showed no significant difference in hospitalisation duration (weighted mean difference 0.01 days, 95% CI –0.63 to 0.65).

The effectiveness of nirsevimab varied by country. The highest effect was observed in the United States (OR 0.07, 95% CI 0.02–0.28), followed by Spain (OR 0.17) and France (OR 0.24). Differences in health-care delivery, immunisation strategies, and prioritisation of high-risk infants may have contributed to these variations.

Subgroup analysis by infant age showed a slightly greater treatment effect in infants older than 3 months (OR 0.19) compared to those 3 months or younger (OR 0.24). The study did not find statistically significant differences in outcomes based on study design (cohort vs. case-control).

The authors noted that “nirsevimab is highly effective in preventing RSV-related outcomes in infants,” with consistent results across most sensitivity analyses. They also pointed out that the protection offered by nirsevimab did not appear to influence the duration of hospital stays among those who were hospitalised.

The study concluded that real-world data support the continued use of nirsevimab to reduce RSV-related health-care use in infants during their first year of life. Future research is needed to understand the broader health and economic impacts and to explore differences in effectiveness by gestational age, comorbidities, or timing of immunisation.

References:

1. Sumsuzzman DW, Wang Z, Langley JM, Moghadas SM. Real-world effectiveness of nirsevimab against respiratory syncytial virus disease in infants: a systematic review and meta-analysis. Lancet Child Adolesc Health 2025. https://doi.org/10.1016/ S2352-4642(25)00093-8

2. The Lancet. The Lancet Child & Adolescent Health: Monoclonal antibody nirsevimab provides strong real-world protection against severe RSV in infants, suggests meta-analysis. The Lancet. Press release. May 1, 2025. Accessed May 1, 2025.