To assess the effectiveness of the monoclonal antibody nirsevimab (Beyfortus®) in addressing the RSV bronchiolitis epidemic during the 2023–2024 season, two complementary studies were conducted in collaboration between the Institut Pasteur and Santé publique France: first, an assessment of the real-world efficacy of nirsevimab against cases of RSV bronchiolitis admitted to intensive care, and second, a modeling study to evaluate the impact in terms of hospitalizations averted.

The results of these two studies agree on the positive impact of nirsevimab on the bronchiolitis epidemic, with a significant decrease in the number of infants hospitalized and an estimated real-world treatment efficacy of between 76% and 81% for infants admitted to intensive care. Furthermore, the administration of nirsevimab prevented approximately 5,800 hospitalizations for bronchiolitis following visits to the emergency department between September 15, 2023, and January 31, 2024, in mainland France.

The real-world efficacy of nirsevimab (Beyfortus®) is estimated to be between 76% and 81% in preventing cases of RSV bronchiolitis admitted to intensive care

Each year, Santé publique France monitors the epidemiological trends of bronchiolitis. For the 2023/2024 season, a pilot surveillance program for severe cases of bronchiolitis in children under two years of age was implemented in collaboration with the PICURe (Pediatric Intensive Care Unit Registry) network, which allowed for the participation of volunteer pediatric or neonatal intensive care units. This surveillance is coordinated in each region by the regional units of Santé publique France. It has made it possible, in particular, to identify the viruses involved in severe cases of bronchiolitis¹ as well as to describe severe RSV bronchiolitis cases that received preventive immunization with nirsevimab (Beyfortus®).

Based on these indicators, Santé publique France conducted a case-control study of the real-world efficacy of nirsevimab in preventing RSV bronchiolitis cases admitted to intensive care.

The results of the study, conducted in mainland France from September 15, 2023, to January 31, 2024, involving 288 infants, confirm the real-world efficacy of nirsevimab in preventing severe cases of RSV bronchiolitis requiring admission to the intensive care unit; which is estimated to be between 75.9% (95% confidence interval: 48.5–88.7) and 80.6% (61.6–90.3) depending on the assumptions. These analyses are consistent with the results of an international clinical study involving 8,058 infants.

5,800 hospitalizations for RSV bronchiolitis prevented, according to modeling studies

To assess the impact of nirsevimab administration on the bronchiolitis epidemic, researchers at the Pasteur Institute and epidemiologists from Santé publique France developed a mathematical model characterizing RSV transmission across different age groups, as well as plausible scenarios for the administration of nirsevimab doses, based on data regarding dose deliveries to maternity wards and pharmacies. They calibrated their model using hospital and virological surveillance data from mid-2017 through February 4, 2024, as well as serological data.

They estimate that the administration of nirsevimab prevented 5,800 (95% credibility interval: 3,700–7,800) hospitalizations for RSV bronchiolitis following a visit to the emergency department, including 4,200 (2,900–5,600) among children aged 0 to 2 months, between September 15, 2023, and February 4, 2024, in mainland France. This corresponds to a 23% (16%–30%) reduction in the total number of hospitalizations for RSV bronchiolitis following a visit to the emergency department (35% (25%–44%) among 0–2-month-olds) compared to the scenario without administration. In the baseline scenario with 215,000 doses administered as of January 31, 2024, the efficacy of nirsevimab against hospitalizations for RSV bronchiolitis was estimated at 73% (61%–84%), corresponding to one hospitalization prevented for every 39 (26–54) doses administered.

Download:

• Nirsevimab effectiveness against cases of respiratory syncytial virus bronchiolitis hospitalized in pediatric intensive care units in France, September 2023 - January 2024 [English only]

• Estimates of effectiveness and impact of nirsevimab on hospitalizations for RSV bronchiolitis in metropolitan France, 2023–2024: a modeling study [English only]

Learn more about bronchiolitis

Bronchiolitis, a viral respiratory infection (primarily caused by the Respiratory Syncytial Virus - RSV), is a highly contagious disease that affects infants and young children under two years of age, especially in the fall and winter. Although bronchiolitis is common and usually mild, it can, in some cases—particularly among infants under two months of age—lead to severe complications and require hospitalization. In France, it is estimated that bronchiolitis affects nearly 30% of infants under 2 years of age each winter, representing approximately 480,000 cases per year. Between 2% and 3% of infants under 1 year of age are hospitalized for more severe bronchiolitis each year.

See also:

• Bronchiolitis: Summary of the 2022–2023 Winter Surveillance | Santé publique France

• The latest ARI (Acute Respiratory Infections) bulletin describing bronchiolitis surveillance, published by Santé publique France, includes the summary of the 2023/2024 ARI season.