Respiratory Syncytial Virus (RSV) remains a formidable pathogen, predominantly impacting the very young and the elderly during the distinct winter respiratory virus season spanning from October to April in Europe. Historically, RSV has presented mostly as a mild illness mimicking a common cold, yet in vulnerable populations—particularly infants under six months and adults over 65 years—the infection can escalate dramatically, causing bronchiolitis, pneumonia, or sepsis. These complications frequently necessitate extended hospitalizations, posing substantial clinical and economic burdens on healthcare infrastructures across the continent.
In recent years, significant strides have been made toward mitigating RSV’s impact via immunisation strategies. Since 2022, the European Union has authorised several prophylactic interventions aimed at curbing severe RSV manifestations in at-risk demographics. Among these are long-acting monoclonal antibodies, such as nirsevimab, administered prophylactically to neonates during the critical winter months, and maternal vaccines designed to confer passive immunity to infants through the transplacental transfer of neutralising antibodies. These immunisation approaches represent a pivotal evolution in combating a virus that previously eluded effective preventative measures.
A groundbreaking multicentre case-control study conducted across Belgium, Portugal, and Spain during the 2024/25 RSV season has elucidated the efficacy of nirsevimab immunisation in hospitalised children under two years of age. This landmark investigation, spearheaded by Savulescu et al. and published in Eurosurveillance, involved a robust cohort of over 4,100 hospitalised pediatric patients presenting with severe acute respiratory infections from September 2024 through May 2025. Out of these, 791 children tested positive for RSV and were further analysed alongside a control group of 1,410 RSV-negative children.
The study’s methodology involved classifying infants as immunised if they had received nirsevimab within the designated timeframe before RSV testing, irrespective of dosage or anthropometric variables such as age and weight. The longitudinal analysis underscored a compelling 79% overall reduction in hospitalisation risk attributable to RSV among immunised children versus controls. These findings decisively affirm the protective capability of monoclonal antibody prophylaxis during the RSV season, solidifying its role as a critical public health tool.
One of the more nuanced revelations from the study pertained to the durability of immunisation-induced protection. The protective efficacy of nirsevimab demonstrated a gradual decline over time post-administration. Initially, within the first month following immunisation, efficacy peaked at approximately 85%. However, this figure moderately decreased to 78% between 30 and 89 days, and further declined to 69% beyond the three-month mark. This temporal attenuation emphasizes the importance of understanding immunological kinetics to optimise timing and scheduling of prophylactic interventions.
Focusing on the highest-risk subgroup—newborns aged zero to six months—the effectiveness of immunisation was notably robust, with an 80% reduction in RSV-related hospitalisation rates documented. This demographic is disproportionately vulnerable due to underdeveloped adaptive immunity and heightened susceptibility to severe lower respiratory tract infections. Such efficacy substantiates the public health imperative of prioritising early-life immunisation programs incorporating long-acting monoclonal antibodies.
The authors advocate for continued surveillance and monitoring of immunisation effectiveness in subsequent seasons to better delineate patterns of waning immunity. This ongoing evaluation is critical to adapting public health policies, including potential booster strategies and refining age cohorts targeted by immunisation efforts. Additionally, the surveillance provides valuable data to anticipate RSV epidemic peaks and allocate healthcare resources more effectively.
Immunisation with monoclonal antibodies not only reduces hospital admissions but also mitigates the severe disease burden on infants by interfering with the virus’ ability to bind and enter respiratory epithelial cells, thus neutralising infection at an early stage. This prophylactic mechanism bypasses the need for an active immune response, an advantageous feature in neonates with immature immune systems, thereby ensuring immediate and sustained protection during the peak RSV circulation period.
The European Centre for Disease Prevention and Control (ECDC) continues to play a pivotal role in orchestrating vaccine effectiveness and burden studies, exemplified by funding the VEBIS project underpinning this research. This collaborative effort across multiple countries enhances the generalisability of findings and provides a scientifically rigorous basis to inform EU-wide immunisation guidelines and policy formulation.
Furthermore, the resurgence of RSV activity resembling pre-pandemic seasonal patterns underscores the virus’s persistence and the necessity for sustained immunisation vigilance. Post-pandemic shifts in respiratory virus epidemiology have prompted recalibrations in surveillance and vaccination strategies to address the heightened susceptibility resulting from altered exposure dynamics during COVID-19 related preventative measures.
The implications of this study extend beyond pediatric immunisation, signaling potential advancements in RSV prophylaxis for other vulnerable groups such as the elderly and immunocompromised individuals, where vaccine efficacy remains under active investigation. Monoclonal antibodies, with their targeted specificity and potential for durable protection, may represent an evolving cornerstone in comprehensive RSV disease management.
In summation, the 2024/25 European multicentre study unequivocally demonstrates that immunisation with long-acting monoclonal antibodies significantly diminishes severe RSV illness necessitating hospital admission in infants. The temporal decline in efficacy observed warrants strategic enhancements in immunisation scheduling and ongoing efficacy monitoring. These findings translate into actionable knowledge pivotal for reducing RSV morbidity, alleviating healthcare burden, and safeguarding the most vulnerable populations during the respiratory virus season.
Subject of Research: People
Article Title: 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
News Publication Date: 13-Nov-2025
Web References:
- https://www.eurosurveillance/content/10.2807/1560-7917.ES.2025.30.45.2500816
- https://www.ema.europa.eu/en/medicines/human/EPAR/beyfortus
- https://www.ema.europa.eu/en/search?keywords=Abrysvo
References:
Savulescu Camelia, Ganser Iris, Nicolay Nathalie, et al. 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). DOI: 10.2807/1560-7917.ES.2025.30.45.2500816
Image Credits: Eurosurveillance
Keywords: Respiratory syncytial virus, Immunisation, Monoclonal antibodies, Infants, Infectious diseases, Public health, Epidemiology, Health care, Human health
