Real-World Effectiveness of Universal Nirsevimab on Bronchiolitis Incidence: A Regional Hub-Spoke Surveillance Analysis in Liguria, Italy

Real-World Effectiveness of Universal Nirsevimab on Bronchiolitis Incidence: A Regional Hub-Spoke Surveillance Analysis in Liguria, Italy | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 24 October 2025 V1 Latest version Share on Real-World Effectiveness of Universal Nirsevimab on Bronchiolitis Incidence: A Regional Hub-Spoke Surveillance Analysis in Liguria, Italy Authors : Marcello Mariani 0000-0003-2989-7301 , Giacomo Brisca [email protected] , Marina Francesca Strati , Silvia Buratti , Marta Ferretti , Alberto Gaiero , Valeria Musso , … Show All … , Andrea Gazzolo , Francesca Ginocchio , Katiuscia Zerbini , Diego Minghetti , Maria Franca Corona , Laura De Hoffer , Daniela Pirlo , Emanuela Piccotti , Luca Antonio Ramenghi , Marisa Alberti , Raffaele Spiazzi , Elio Castagnola , and Andrea Moscatelli Show Fewer Authors Info & Affiliations https://doi.org/10.22541/au.176128214.43926638/v1 Published Pediatric Pulmonology Version of record Peer review timeline 276 views 158 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Background: Bronchiolitis represents a major cause of infant hospitalization, predominantly due to respiratory syncytial virus (RSV). Nirsevimab, a long-acting monoclonal antibody targeting the prefusion RSV F protein, has recently been introduced to provide season-long protection against RSV-related lower respiratory tract infection. Methods: We conducted a multicenter, retrospective, population-based study across the Liguria Region (Northwest Italy) to assess the impact and effectiveness of the 2024–2025 nirsevimab immunization campaign. All infants under two years of age hospitalized with acute bronchiolitis between October 2023 and March 2025 were identified through the regional pediatric Hub-and-Spoke network. Data were compared between the pre-nirsevimab (2023–2024) and post-nirsevimab (2024–2025) seasons. Nirsevimab effectiveness (VE) against RSV-positive hospitalization was estimated using a test-negative design. Results: A total of 858 infants were hospitalized with bronchiolitis across the two seasons. Following the introduction of nirsevimab, bronchiolitis hospitalizations decreased from 41.7 to 17.3 per 1000 admissions (Incidence Rate Ratio 0.4; 95% CI: 0.4–0.5; p<0.001), corresponding to a 60% reduction. Estimated nirsevimab effectiveness was 65.0% (95% CI: 39.2–79.8%) against RSV-positive hospitalization. No high-risk infants—those previously eligible for palivizumab—developed severe RSV infection requiring PICU admission. Breakthrough infections occurred in a minority of immunized infants but showed a similar clinical course to non-immunized cases. Conclusions: The introduction of universal nirsevimab immunization in Liguria was associated with a marked reduction in RSV-related hospitalizations, confirming its strong real-world epidemiological impact. These findings support nirsevimab as an effective and feasible public health intervention to substantially reduce the burden of RSV bronchiolitis in infants. Introduction Bronchiolitis is a major global health concern, causing substantial morbidity and mortality in infants and younger children, particularly those under 2 years of age. The disease is most commonly triggered by respiratory syncytial virus (RSV), which is responsible for over 30 million cases of lower respiratory tract infection and more than 3 million hospitalizations annually in children under 5 years worldwide, with approximately 200,000 deaths each year—disproportionately affecting low-income countries 1,2 . In high-income settings, bronchiolitis is the leading cause of infant hospitalization, accounting for up to 15–17% of all pediatric admissions in children younger than 2 years and a significant proportion of emergency department visits 1-3 . After the failure of early vaccine attempts in the 1960s, passive immunoprophylaxis became the mainstay of prevention. The first major advance was the development of RSV intravenous immunoglobulin in the 1990s, followed by the introduction of palivizumab, a humanized monoclonal antibody targeting the RSV F protein, which was licensed in 1998 for the prevention of severe RSV disease in high-risk pediatric populations 4,5 . Despite its impact in reducing RSV hospitalizations among high-risk infants, the majority of severe RSV cases occur in otherwise healthy infants who do not qualify for palivizumab, highlighting the need for broader and more cost-effective preventive strategies 1,6 . Nirsevimab immunization was developed as a long-acting monoclonal antibody targeting the prefusion F protein of RSV, with the goal of providing season-long protection against RSV-associated lower respiratory tract infection in infants through a single intramuscular dose 7,8 . Nirsevimab received its first regulatory approval in Europe in November 2022, with the European Medicines Agency and the UK’s Medicines and Healthcare products Regulatory Agency authorizing its use for the prevention of RSV lower respiratory tract disease in neonates and infants during their first RSV season 8 . Implementation in Europe began in the autumn of 2023, with national immunization programs in countries such as France, Germany, and the United Kingdom integrating nirsevimab into routine practice for infants entering their first RSV season 9,10 . In Italy, initial implementation began regionally, with Valle d’Aosta pioneering universal prophylaxis for all newborns and infants during the 2023–2024 RSV season; this program demonstrated a marked reduction in RSV bronchiolitis hospitalizations, with no hospitalizations among immunized infants and a favorable safety profile 11 . In the 2024–2025 season, nirsevimab was incorporated into the national immunization schedule, but regional differences in timing and coverage persisted, as seen in the regions of Lazio and Lombardy, where delayed or partial rollout led to variable reductions in RSV-related hospitalizations 12,13 . While recent single-center analyses from Italy provide valuable real-world effectiveness data regarding nirsevimab’s impact on individual patient outcome 12,13 , we aimed to advance the existing evidence by providing robust surveillance data on hospitalized bronchiolitis incidence, leveraging a Regional registry collected through a Hub-Spoke pediatric network. Materials and Methods The Liguria Region is situated in Northwest Italy and has a population of approximately 1.5 million inhabitants. Pediatric hospital care across the region is guaranteed by a structured Hub-and-Spoke network. The network’s Hub center is the IRCCS Istituto Giannina Gaslini, a third-level pediatric tertiary care center located in the capital city of Genoa. This is complemented by four peripheral Spoke centers, which are located in the cities of Imperia, Savona, Lavagna, and La Spezia. This configuration ensures that all pediatric hospital assistance within the region is provided by a unified system. Each Spoke is equipped with a pediatric emergency service responsible for the acute clinical management of patients. If the clinical condition allows, the patient is managed directly at the Spoke facility (as each one has inpatient pediatric beds); otherwise, patients requiring a higher intensity of care are centralized to the Hub. For the diagnostic and therapeutic management of bronchiolitis, clinicians follow a standardized regional protocol jointly developed by multiple stakeholders. We conducted a multicenter, retrospective study across the entire Liguria Region pediatric network. We retrospectively identified all patients younger than two years hospitalized with a primary diagnosis of acute bronchiolitis (International Classification of Diseases, Ninth Revision, Clinical Modification [ICD-9-CM] code 466.19). Data collection spanned two RSV seasons, from October 1, 2023, to March 31, 2024, and from October 1, 2024, to March 31, 2025. Demographic and clinical data, including patient age, requirement for respiratory support, microbiological findings, and outcome, were systematically collected. RSV identification was performed via either Polymerase Chain Reaction (PCR) or point-of-care rapid antigen testing on nasopharyngeal aspirates. Data were aggregated and compared across two complete seasons: 2023–2024 and 2024–2025. Nirsevimab Immunization Program In alignment with National recommendations, the Liguria Region defined the introduction of a Nirsevimab immunization program for the 2024/2025 RSV season via Regional Council Resolution (DGR) 788/2024. The program was initiated on December 5, 2024, targeting infants born on or after November 1, 2024, with a catch-up phase extended to infants born within the previous 100 days (i.e., those born after July 24, 2024). Descriptive statistics summarized demographic, clinical, and laboratory characteristics, reported as median and interquartile range (IQR) for continuous variables, and as frequencies and percentages for categorical variables. Group comparisons utilized appropriate tests based on data distribution, including the Student’s t-test or Mann-Whitney U test for continuous variables, and the Pearson’s chi-square test or Fisher’s exact test for categorical variables. Incidence changes between the 2023–2024 and 2024–2025 seasons were assessed using the Incidence Rate Ratio (IRR). A p-value less than 0.05 was considered statistically significant. Nirsevimab effectiveness (VE) against hospitalization for RSV-positive bronchiolitis was estimated using a test-negative design (TND) 14,15 , comparing immunization status between infants hospitalized with RSV-positive bronchiolitis (cases) and those hospitalized with RSV-negative bronchiolitis (controls). VE was derived from the calculated Odds Ratio (OR) using the formula VE = (1 - OR) x 100. A p-value of less than 0.05 was considered statistically significant. This study was conducted in accordance with the Helsinki Declaration. According to Italian legislation, the study did not need ethical approval, as it was a purely observational retrospective study on routinely collected anonymous data. Furthermore, it was not possible to request informed consent for participation in the study, given the nature of the study. In any case, consent to completely anonymous use of clinical data for research/epidemiological purposes is requested by the clinical routine at the time of admission/diagnostic procedure. Results Overall, we identified 858 patients (female 373, 43%; male 485, 57%) hospitalized with acute bronchiolitis across the two study seasons. The median age of the cohort was 104 days (Interquartile Range [IQR], 56–177). Epidemiological impact Data were distributed between the pre-nirsevimab season (2023-2024, n=552) and the post-nirsevimab season (2024-2025, n=306). The primary analysis showed a significant decrease in the incidence of hospitalized bronchiolitis. Figure 1 shows bronchiolitis regional trends over the study period. As summarized in Table 1, the incidence rate dropped from 41.7 per 1000 total admissions (95% CI: 38.3–45.3) in the 2023-2024 season to 17.3 per 1000 total admissions (95% CI: 15.5–19.4) in the 2024-2025 season. This resulted in an Incidence Rate Ratio (IRR) of 0.4 (95% CI: 0.4–0.5; p < 0.001), reflecting a 60% reduction in hospitalized bronchiolitis incidence across the Liguria Region. Comparison of the two cohorts revealed significant differences in median age (p < 0.001), which was higher in the 2024-2025 season. Conversely, the rate of pre-existing disease did not change (p=0.535). From an etiological point of view, the overall RSV positivity rate remained comparable between the two seasons (p > 0.05). Supplementary Table 1 shows the etiological characterization of children with RSV-negative bronchiolitis across the two different seasons. No differences were noted except for a slight, although significant, increase in Influenza A/B bronchiolitis in the last season. Comparing the clinical severity parameters of the two cohorts of hospitalized patients, we did not observe any statistically significant difference in the need for respiratory support, the median length of stay, or the need for invasive ventilation. Nirsevimab effectiveness As illustrated in Figure 2, immunization with nirsevimab showed a highly significant protective effect (χ 2 = 14.6; p < 0.001). The calculated Odds Ratio (OR) was 0.350 (95% CI: 0.202–0.608), which translates to an estimated nirsevimab effectiveness (VE) of 65.0% (95% CI: 39.2%–79.8%) against hospitalization for RSV-positive bronchiolitis. Among the 73 admitted patients (23.7%) who had received nirsevimab, 39 children (53%) were RSV positive. Of these, two (2.7%) were admitted to the PICU, but none required invasive ventilation. Critically, no high-risk infants - those who historically qualified for palivizumab - developed severe RSV infection requiring PICU admission. However, while nirsevimab proved highly effective at preventing hospitalization, its impact on the clinical course of breakthrough infections appeared limited. Specifically, when comparing immunized versus non-immunized infants hospitalized with RSV-positive bronchiolitis, we found no significant differences in the need for respiratory support, intensity of care, or length of hospital stay. Clinical characteristics comparing these two groups are detailed in Supplementary Table S2. Discussion The current study provides real-world evidence from an Italian regional pediatric network (Liguria) demonstrating the important epidemiological impact and estimated effectiveness of the nirsevimab immunization program on hospitalized acute bronchiolitis during the 2024–2025 season. Our findings must be interpreted within the context of altered post-pandemic viral circulation. As noted worldwide, and also reported in our setting 16,17 , a significant shift in bronchiolitis epidemiology, characterized by a marked increase in RSV detection rates and a corresponding surge in the need for respiratory support, was observed after the SARS-CoV-2 pandemic. The 2022–2023 season, in particular, was marked by an unprecedented increase in disease severity in our Region 18 , confirming that a high disease burden was prevalent in Italy just before the nirsevimab introduction 19 . The core finding of our surveillance is the remarkable 60% reduction (IRR 0.4) in the overall incidence of hospitalized bronchiolitis following the introduction of the immunization program, confirming a substantial epidemiological impact of the nirsevimab immunization in Liguria. Our results suggest that the implementation of the nirsevimab program through the Liguria healthcare system was critical in mitigating the severe post-pandemic epidemiological burden. The estimated nirsevimab effectiveness of 65.0% against RSV-positive hospitalization further validates the protective impact of the program. By comparing with early real-world data reported from other major Italian centers 13 , our results align with robust real-world protection, despite potential slight variations due to campaign timing and coverage. As previously stated, in Liguria, the regional campaign began relatively late in the 2024–2025 season due to initial difficulties in drug availability. The program was officially launched in early December, some weeks after the onset of the RSV epidemic period. This delay likely limited the overall effectiveness observed in our setting compared to the findings of other authors who reported a reduction in RSV hospitalizations of over 80% in their setting 20,21 . It is therefore reasonable to expect that in future seasons, with earlier campaign initiation better aligned with RSV epidemiology, even greater reductions in RSV-related hospitalizations could be achieved. Similarly, other Italian Regions observed only a 43% overall decrease in RSV admissions, a suboptimal result attributed to a delayed campaign start and difficulties in achieving adequate coverage, particularly in the catch-up population 12 . Taken together, these findings emphasize the importance of timely program logistics as a key determinant of real-world effectiveness in large-scale immunization strategies. Notably, our cohorts showed a significant change in the age profile, with the median age of hospitalized infants shifting from 93.5 days to 152 days after the immunization program. This age shift is an expected result of a selective immunization strategy targeting newborns and younger infants, effectively pushing the burden of hospitalization toward the older, non-immunized segment of the pediatric population. This finding underscores the need for continued surveillance to evaluate the impact on older children. Crucially, in terms of clinical outcomes, we observed no significant differences in the overall need for respiratory support or length of stay between the two seasons, suggesting that while the total volume of severe cases was dramatically reduced, the clinical course for those still requiring hospitalization remained largely stable. Nevertheless, the observed reduction in hospitalizations is particularly relevant given that the post-pandemic literature points to an increased need for intensive management strategies with a corresponding dramatic increase in associated costs 22 . Our data support the growing evidence that proactive immunization is a superior public health strategy that effectively reduces the need for such resource-intensive measures by significantly limiting the number of cases. In our cohort, the protective effect of nirsevimab was also confirmed among infants with pre-existing conditions traditionally considered at higher risk for severe RSV disease. This finding is particularly relevant because these subgroups historically represented the main target population for monthly palivizumab prophylaxis 23 , reinforcing the real-world effectiveness of this monoclonal antibody across diverse clinical conditions. Recent pharmacokinetic and safety data from the MEDLEY trial demonstrated comparable serum exposure and tolerability between high-risk infants receiving nirsevimab and those receiving palivizumab, supporting its use in these populations 24 . Our data contribute additional real-world evidence supporting nirsevimab as an effective and logistically advantageous alternative to palivizumab for preventing severe RSV disease in vulnerable infants. An important finding from our study is the presence of a non-negligible proportion of infants who, despite having received nirsevimab immunization, were still infected with RSV and required hospitalization for bronchiolitis. In these cases, the clinical course and severity parameters appeared comparable to those of non-immunized patients. This observation is consistent with emerging literature indicating that breakthrough RSV infections after nirsevimab do occur, reflecting the high but not absolute protection provided by the antibody. Real-world effectiveness studies have reported protection rates of approximately 70–80% against RSV infection and over 80% against hospitalization, confirming substantial but incomplete immunity 20,21,25 . Importantly, some reports indicate that effectiveness may decline over time, with protection decreasing from about 79% at two weeks to 55% by 14 weeks post-administration, suggesting that waning antibody levels may contribute to breakthrough cases 26 . Possible explanations for these infections include interindividual variability in pharmacokinetics (related to body weight, gestational age, or timing of administration relative to viral exposure), differences in local RSV circulation dynamics, or intrinsic immunological factors leading to “non-responder” status in a small subset of infants. Although detailed analyses of such subgroups remain scarce, our findings reinforce the need for future prospective studies to identify these infants and better understand determinants of reduced response. In parallel, it is essential to monitor potential shifts in the viral epidemiology of bronchiolitis. The decline in RSV circulation following widespread immunization could, through ecological replacement, lead to an increased relative burden of bronchiolitis caused by other respiratory viruses such as rhinovirus, metapneumovirus, or parainfluenza virus. Although this phenomenon has not yet been observed in our setting, continuous virological surveillance and etiological characterization of bronchiolitis cases will remain critical to fully understand post-nirsevimab dynamics and to ensure an appropriate public health response. This study has several limitations inherent to its design. First, its retrospective and observational nature means we cannot exclude the possibility of unmeasured residual confounding factors that may have influenced the effectiveness estimates. Second, while the Liguria Hub-and-Spoke network ensured comprehensive regional data collection, the single-region scope limits direct generalization to the entire national context, particularly given the documented regional variations in campaign initiation and coverage across Italy. Finally, the use of a test-negative design means that infants who presented with less severe bronchiolitis and were not hospitalized, or who did not seek medical care, were excluded from the analysis. This potential selection bias may affect the overall generalizability of the effectiveness estimate to the entire infant population. Despite these limitations, the main strength of our analysis is that it provides early, robust, population-level evidence on the epidemiological impact and effectiveness of nirsevimab in an Italian regional healthcare setting, offering valuable insights for public health decision-making. Conclusions The universal nirsevimab immunization program introduced in the Liguria Region during the 2024–2025 RSV season led to a 60% reduction in bronchiolitis hospitalizations, confirming its strong epidemiological impact. The estimated effectiveness of 65% against RSV-positive hospitalization aligns with international real-world data. Notably, no severe RSV cases occurred among high-risk infants, supporting nirsevimab’s efficacy in populations previously eligible for palivizumab. Although breakthrough infections were observed, their clinical course was comparable to that of non-immunized patients. Continuous surveillance remains essential to monitor potential shifts in viral etiology and long-term protection. Overall, these findings highlight nirsevimab as an effective and feasible public health strategy to substantially reduce the burden of RSV-related hospitalizations in infants. References: 26. Dalziel SR, Haskell L, O’Brien S, Borland ML, Plint AC, Babl FE, Oakley E. 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PMID: 40063022; PMCID: PMC11894488.Wilkins D, Wählby Hamrén U, Chang Y, Clegg LE, Domachowske J, Englund JA, Muller WJ, Leach A, Kelly EJ, Villafana T. RSV Neutralizing Antibodies Following Nirsevimab and Palivizumab Dosing. Pediatrics. 2024 Nov 1;154(5):e2024067174. doi: 10.1542/peds.2024-067174. PMID: 39350745. Image legends Figure 1: Bronchiolitis hospitalization trend in Liguria Region over the seasons 2023-2024 and 2024-2025 Figure 2: Distribution of Nirsevimab immunized patients by RSV positivity Supplementary Material File (table 1.docx) Download 16.48 KB Information & Authors Information Version history V1 Version 1 24 October 2025 Peer review timeline Published Pediatric Pulmonology Version of Record 5 May 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords bronchiolitis immunization program nirsevimab real-world effectiveness respiratory syncytial virus Authors Affiliations Marcello Mariani 0000-0003-2989-7301 Istituto Giannina Gaslini View all articles by this author Giacomo Brisca [email protected] Istituto Giannina Gaslini View all articles by this author Marina Francesca Strati Istituto Giannina Gaslini View all articles by this author Silvia Buratti Istituto Giannina Gaslini View all articles by this author Marta Ferretti Istituto Giannina Gaslini View all articles by this author Alberto Gaiero Istituto Giannina Gaslini View all articles by this author Valeria Musso Istituto Giannina Gaslini View all articles by this author Andrea Gazzolo Istituto Giannina Gaslini View all articles by this author Francesca Ginocchio Istituto Giannina Gaslini View all articles by this author Katiuscia Zerbini Istituto Giannina Gaslini View all articles by this author Diego Minghetti Istituto Giannina Gaslini View all articles by this author Maria Franca Corona Istituto Giannina Gaslini View all articles by this author Laura De Hoffer Istituto Giannina Gaslini View all articles by this author Daniela Pirlo Istituto Giannina Gaslini View all articles by this author Emanuela Piccotti Istituto Giannina Gaslini View all articles by this author Luca Antonio Ramenghi Istituto Giannina Gaslini View all articles by this author Marisa Alberti Istituto Giannina Gaslini View all articles by this author Raffaele Spiazzi Istituto Giannina Gaslini View all articles by this author Elio Castagnola Istituto Giannina Gaslini View all articles by this author Andrea Moscatelli Istituto Giannina Gaslini View all articles by this author Metrics & Citations Metrics Article Usage 276 views 158 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Marcello Mariani, Giacomo Brisca, Marina Francesca Strati, et al. 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