Study in Europe: monoclonal antibodies effectively prevent respiratory syncytial virus in infants

Respiratory syncytial virus (RSV) is a common respiratory virus which usually causes mild, cold-like symptoms during the respiratory virus season. However, especially among young infants under six months of age or people over 65, RSV infection might turn more severe. Health complications associated with RSV include bronchiolitis (inflammation of the small airways in the lung), pneumonia or even sepsis which can require prolonged hospital stays. 

Immunisation products to prevent RSV disease in infants and older adults have been authorised in the European Union since 2022. These include long-acting monoclonal antibodies are given to newborns during the winter season as well as vaccines for pregnant people to protect newborns during their first winter season after birth. [1] The European RSV season has returned to pre-pandemic seasonality, lasting from around October to April.

Immunisation of infants effectively prevented hospitalisation related to RSV
In their rapid communication published in Eurosurveillance, Savulescu et al. looked at data from three European countries which had RSV immunisation programmes in place during the winter season 2024/25 to establish the effectiveness of the long-acting monoclonal antibody (nirsevimab) against RSV infection. [2]

For their case-control study among children below the age of two years, the authors screened data from 4,102 hospitalised children in Belgium, Portugal and Spain between September 2024 and May 2025. Of those, 791 children who had been tested positive for RSV were included in the study analysis and 1,410 children who tested negative for RSV in the control group.

In the study, children were considered immunised if they received nirsevimab between September 2024 and May 2025 before testing, regardless of the dose, or their age and weight at the time of immunisation.

Protection levels decline incrementally several months after immunisation
Based on their analysis, Savulescu et al. conclude that “immunisation of children after birth effectively prevented RSV-related hospitalisation in children during the 2024/25 European winter season”. Among 2,201 children the pooled overall immunisation effectiveness was at 79%, i.e. receiving long-acting monoclonal antibodies significantly reduced the risk of hospitalisation due to RSV infection in this age group.

However, the level of protection declined over time following immunisation: starting from 85% during the first month (<30 days) after immunisation to 78% during days 30 to 89 after immunisation and 69% after three months (90 to 215 days) from immunisation.

Among infants aged 0–6 months, i.e. the newborns at highest risk for severe RSV illness, the overall effectiveness of immunisation with long-acting monoclonal antibodies was 80%. The authors highlight that “effectiveness by time since immunisation needs monitoring in future seasons.”

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Notes to editors/reference

[1] Among the authorised RSV immunisation products are long-acting monoclonal antibody product administered directly to neonates and infants (Beyfortus/nirsevimab) and one maternal vaccine to provide protection to infants through transplacental transfer of RSV neutralising antibodies (Abrysvov).
[2] Savulescu Camelia, Ganser Iris, Nicolay Nathalie, Lajot Adrien, Campos Sandra, Martínez-Baz Iván, Rodrigues Ana Paula, Vandromme Mathil, Cara-Rodríguez Marta, Echeverría Aitziber, Gaio Vânia, Parsy Marie-Pierre, Garrido Ana Roldan, Castilla Jesús, Guiomar Raquel, Bacci Sabrina, Rose Angela MC, the VEBIS hospital network RSV IE group. Effectiveness of long-acting monoclonal antibodies against laboratory-confirmed RSV in children aged < 24 months and hospitalised for severe acute respiratory infection, European pilot study, 2024 to 2025. Euro Surveill. 2025;30(45), Available from: https://www.eurosurveillance/content/10.2807/1560-7917.ES.2025.30.45.2500816
This study was funded by European Centre for Disease Prevention and Control through Vaccine Effectiveness, Burden and Impact Studies (VEBIS) Lot 1 framework contract ECDC/2021/016.
[3] Marking the start of the 2025/26 season for respiratory diseases in Europe, ECDC issues a rapid scientific advice on effective ways to mitigate the impact of RSV disease in infants through immunisation interventions.  Available from: https://www.ecdc.europa.eu/en/publications-data/rapid-scientific-advice-protecting-infants-against-respiratory-syncytial-virus