In this case-control study published in the weekly epidemiological bulletin of the Centers for Disease Control and Prevention, the national surveillance network New Vaccine Surveillance Network (NVSN) was used to evaluate the effectiveness of nirsevimab against hospitalizations for RSV bronchiolitis between October 1, 2023 and February 29, 2024. The data came from 4 cities participating in the surveillance (Houston, Nashville, Pittsburgh, Seattle).
Out of over 1,000 eligible babies hospitalized for bronchiolitis, 699 were included with almost half (47%) aged between 0 and 2 years. In this cohort, 58% had a confirmed RSV infection by PCR (407 “cases”) and 42% had a negative test result (292 “controls”); in total, 8% had received a nirsevimab injection at least 7 days before the onset of symptoms (time elapsed since injection ranged from 7 to 127 days, with a median duration of 45 days).
Among confirmed RSV bronchiolitis cases, 1% had been immunized with nirsevimab. This proportion was 18% among control cases. Thus, the statistical analysis – adjusted for variables such as age at inclusion, inclusion site, and presence of one or more comorbidities at risk of severe bronchiolitis – estimated that nirsevimab had a 90% effectiveness against hospitalizations for RSV bronchiolitis (95% CI: 75 to 96).
This effectiveness is higher than that observed in the Harmony clinical trial phase III, published in the NEJM
in December 2023 (83%, but this study had a longer follow-up of 3 months). The authors of the American report emphasize that, taking into account the entire RSV season, the expected effectiveness of nirsevimab should be lower than their estimation as antibody levels from this immunization decrease over time. However, they point out that in the clinical trial Melodytesting the molecule’s effectiveness against symptomatic infection requiring medical management, nirsevimab remained highly effective up to 150 days after injection – which is consistent with its long half-life of 63 to 73 days.
Finally, although surveillance should continue to provide more robust results – especially with longer follow-up and a larger number of immunized children, the authors conclude that this initial estimation supports the current recommendation to immunize infants with nirsevimab to prevent hospitalizations due to RSV bronchiolitis.
In Francepreliminary data mentioned by Public Health France on nearly 250,000 immunized children in maternity wards suggests an efficacy ranging from 70 to 80%, but official numbers have not yet been published.
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2024-03-19 19:13:51
