Nirsevimab Rollout Cuts Infant Bronchiolitis Across Europe, But Coverage Is Everything

A sweeping analysis of pediatric emergency department data from 12 European countries has found that the rollout of nirsevimab, the long-acting monoclonal antibody designed to protect infants against respiratory syncytial virus, was associated with dramatic reductions in bronchiolitis visits during the 2023–2024 respiratory season. But the study, published in PLOS Medicine, also carries a sobering caveat: the size of the benefit varied enormously from one region to another, and that variation tracked closely with how many eligible infants actually received the shot.

Respiratory syncytial virus, or RSV, is the leading cause of bronchiolitis, a viral infection of the small airways that sends hundreds of thousands of infants to hospitals across Europe each winter. For decades, pediatricians had little to offer beyond supportive care, since a previous antibody product, palivizumab, was reserved for high-risk infants because of its high cost and the need for monthly injections. Nirsevimab changed that calculus. A single dose provides protection lasting at least five months, covering an entire RSV season, and clinical trials demonstrated strong efficacy against medically attended lower respiratory tract infection in healthy term and preterm infants alike.

When nirsevimab finally became available in Europe in late 2023, national and regional health authorities adopted it at different speeds and with different strategies. Some countries launched universal immunization programs targeting all infants entering their first RSV season, while others restricted initial campaigns to infants born during or just before the season, and a few had no program at all during the study window. This patchwork of implementation created an unintended natural experiment, and a large consortium of European pediatric emergency researchers seized the opportunity to measure what happened.

The research team, led by Léa Lenglart and Naim Ouldali with the EPISODES Study Group, assembled routinely collected data from 27 pediatric emergency departments spanning 12 European countries, covering the period from January 2018 through March 2024. They included every case of bronchiolitis diagnosed in infants younger than 12 months, yielding a total of 107,088 episodes. Regions were classified as intervention or control depending on whether nirsevimab had been implemented, and immunization coverage figures were drawn from official regional reports. Urinary tract infections in the same age group served as a control outcome, a standard technique to detect whether observed changes reflect a real intervention effect or some unrelated shift in emergency department attendance.

The analytical approach was a controlled interrupted time-series design, a method well suited to evaluating population-level interventions introduced at a known point in time. The models accounted for underlying seasonality and pre-existing temporal trends, allowing the researchers to estimate how much of the change in monthly bronchiolitis cases during the intervention period, from October 2023 through March 2024, could be attributed to the antibody program rather than to the ordinary rhythm of winter epidemics. Estimates were stratified by age group, separating infants aged 0 to 3 months, who face the highest risk of severe disease, from those aged 3 to 12 months.

The results were striking in both their magnitude and their heterogeneity. Reductions in bronchiolitis visits ranged from essentially no change, with one estimate of plus 0.6 percent, to a remarkable 61 percent decline, with a confidence interval stretching from minus 67.8 to minus 35.2 percent. Infants in the youngest age band generally showed the largest reductions, consistent with the biology of RSV, since the highest rates of hospitalization and intensive care admission occur in the first months of life. Meanwhile, bronchiolitis trends in control regions without nirsevimab programs remained stable, and urinary tract infection visits showed no comparable decline, strengthening the argument that the observed reductions were tied to the immunization campaigns rather than a general drop in emergency presentations.

Perhaps the most consequential finding was the strength of the statistical relationship between coverage and benefit. Across regions and age groups, the correlation between nirsevimab coverage and the percentage reduction in bronchiolitis visits was negative and pronounced, with a correlation coefficient of minus 0.86 and a p-value of 0.001. In practical terms, regions that immunized a larger share of their infant population saw proportionally larger drops in emergency department visits. The relationship held across both age strata, suggesting that the antibody’s protective effect translated into population benefit wherever it reached enough infants, whether they were immunized at birth during the season or through catch-up campaigns targeting older babies.

Why did coverage differ so much across Europe? The study points to implementation choices as the decisive variable. Countries and regions that embedded nirsevimab into routine newborn care, offering the dose in maternity wards before discharge, achieved far higher uptake than those relying on later appointments or opportunistic vaccination during other visits. Administrative hurdles, supply constraints, differences in eligibility criteria, and varying levels of parental and clinician acceptance all likely contributed. The authors emphasize that the antibody’s intrinsic efficacy, already demonstrated in randomized trials, was never in question; what varied was the fraction of the infant population that actually received it in time to be protected when the virus arrived.

The researchers are careful about the limits of their design. As an ecological study, it examines populations rather than individuals, so causality cannot be definitively inferred. Unmeasured regional differences, such as changes in testing practices, health-seeking behavior, viral circulation intensity, or concurrent public health measures, may have contributed to some of the observed variability. The interrupted time-series models adjust for seasonality and trend, and the stability of control outcomes argues against simple artifacts, but the authors acknowledge that residual confounding remains possible. Still, the consistency of the findings across 27 departments, multiple countries, and two age groups, together with the tight coverage-response relationship, makes a compelling case that immunization coverage was a key driver of the reductions.

The implications for health policy are immediate. RSV prevention has entered a new era, with nirsevimab programs now expanding and maternal RSV vaccines adding another layer of protection for newborns. This study demonstrates that the public health return on these interventions depends not only on the biology of the product but on the machinery of delivery. Regions that achieved high coverage among both season-born and out-of-season infants reaped reductions in bronchiolitis visits of a scale rarely seen for a respiratory disease, easing pressure on emergency services during the busiest weeks of the winter. For health authorities weighing how to structure their own programs, the European experience of 2023–2024 offers a clear lesson: the fastest, broadest, and earliest possible reach into the infant population is what converts a highly effective antibody into a visible decline in disease. As nirsevimab programs mature and additional seasons of data accumulate, researchers will be watching to see whether the coverage-response relationship holds, whether protection extends to severe outcomes such as intensive care admission, and whether the substantial regional variability in implementation narrows as systems learn from one another.

Subject of Research: Association between nirsevimab immunization coverage and reductions in infant bronchiolitis emergency department visits across 12 European countries

Article Title: Association between nirsevimab coverage and pediatric emergency department visits for bronchiolitis in 12 European countries: An interrupted time-series analysis

Article References: Lenglart, L., Titomanlio, L., Alberti, I., Almeida, L., Aupiais, C., Akyüz Özkan, E., Barrett, M., Basmaci, R., Birbilen, A., Borensztajn, D., Schönenberger, C. B., Bressan, S., Buonsenso, D., Campos, T., Castanhinha, S., Chiaretti, A., Claret, G., De Zan, F., Durnin, S., … the EPISODES Study Group (2026). Association between nirsevimab coverage and pediatric emergency department visits for bronchiolitis in 12 European countries: An interrupted time-series analysis. PLOS Medicine, 23(9), e1005223. https://doi.org/10.1371/journal.pmed.1005223

Image Credits: AI Generated

DOI: 10.1371/journal.pmed.1005223

Keywords: nirsevimab, RSV, bronchiolitis, infants, pediatric emergency departments, immunization coverage, interrupted time-series, Europe, PLOS Medicine, monoclonal antibody, respiratory syncytial virus, public health