Virus detection in infants with bronchiolitis and children with Lower Respiratory Tract Infections after nirsevimab introduction: a retrospective comparative two-season study

Virus detection in infants with bronchiolitis and children with Lower Respiratory Tract Infections after nirsevimab introduction: a retrospective comparative two-season study | 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 Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed.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. 9 June 2025 V1 Latest version Share on Virus detection in infants with bronchiolitis and children with Lower Respiratory Tract Infections after nirsevimab introduction: a retrospective comparative two-season study Authors : Domenico Paolo La Regina 0000-0002-8297-8920 , M. Spatuzzo , Enea Bonci , Raffaella Nenna 0000-0001-8880-3462 , Laura Petraca 0000-0002-2430-2981 , Maria Giulia Conti , Enrica Mancino 0000-0003-2756-0094 , Alessandra Pierangeli 0000-0003-0633-360X , Guido Antonelli 0000-0002-2533-2939 , and Fabio Midulla 0000-0001-7476-5266 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.174943635.57904021/v1 315 views 215 downloads Contents Abstract Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Introduction Respiratory Syncytial Virus (RSV) is the leading cause of lower respiratory tract infections (LRTIs) in children. In 2024, the monoclonal antibody nirsevimab became available in Italy for all newborns during their first RSV season. Aim To evaluate whether there were changes in the circulation of respiratory viruses among hospitalized children under five years of age with viral LRTI. Secondarily, whether the introduction of nirsevimab influenced the hospitalization rate. Methods A retrospective study was conducted, comparing two epidemic seasons: pre-nirsevimab (2023–2024) and post-nirsevimab (2024–2025).All children under five years of age hospitalized for viral LRTIs were included and divided in bronchiolitis and “other-LRTIs”. All hospitalized patients underwent nasopharyngeal swab testing using multiplex-PCR for respiratory viruses’ detection. Results A total of 278 patients were enrolled. RSV circulation decreased in the post-nirsevimab season, both in the bronchiolitis and LRTI groups, although it remained the main cause of infections. In the post-nirsevimab season, detections of influenza virus and bocavirus increased in the bronchiolitis cohort (p-values 0.02 and 0.01), while human metapneumovirus and coronaviruses were more frequently identified in children with LRTIs (p-values 0.08 and 0.03). Regarding infants with bronchiolitis, we showed a 59% reduction in hospitalizations rate (both in paediatric ward and paediatric intensive care) ( p -value < 0.001). Children hospitalized for bronchiolitis were older than the previous season (p < 0.001). No differences were observed in the hospitalization rate among infants with viral LRTI. Conclusions Following nirsevimab introduction, circulation of non-RSV respiratory viruses increased, while hospitalizations and ICU admissions for bronchiolitis significantly decreased. Virus detection in infants with bronchiolitis and children with Lower Respiratory Tract Infections after nirsevimab introduction: a retrospective comparative two-season study D.P. La Regina, 1 M. Spatuzzo, 1 E. Bonci, 1 R. Nenna, 1 L. Petrarca, 1 M.G. Conti, 1 E. Mancino, 1 A. Pierangeli, 2 G. Antonelli, 2 Fabio Midulla 1 1 Department of Maternal Child and Urological Sciences, Sapienza University of Rome, Rome, Italy 2 Department of Virology and Microbiology, Sapienza University of Rome, Rome, Italy Running title : viruses, bronchiolitis and other LRTIs after nirsevimab introduction Correspondence Prof Fabio Midulla , Department of Maternal Child and Urological Sciences, Sapienza University of Rome, Rome, Italy Email: [email protected] Tel: + 0649979363 Financial Disclosure All the authors declare that they have no financial relationships relevant to this article to disclose. Funding Source This work was supported by EU funding within the NextGenerationEU-MUR PNRR Extended Partenership initiative on Emerging Infectious Diseases (project no. PE00000007, INF-ACT) Conflict of Interest All the authors declare that they have no potential conflicts of interest to disclose. Word count of the abstract: 249/250 Word count of the manuscript: 2425/3500 Abstract Introduction Respiratory Syncytial Virus (RSV) is the leading cause of lower respiratory tract infections (LRTIs) in children. In 2024, the monoclonal antibody nirsevimab became available in Italy for all newborns during their first RSV season. Aim To evaluate whether there were changes in the circulation of respiratory viruses among hospitalized children under five years of age with viral LRTI. Secondarily, whether the introduction of nirsevimab influenced the hospitalization rate. Methods A retrospective study was conducted, comparing two epidemic seasons: pre-nirsevimab (2023–2024) and post-nirsevimab (2024–2025).All children under five years of age hospitalized for viral LRTIs were included and divided in bronchiolitis and “other-LRTIs”. All hospitalized patients underwent nasopharyngeal swab testing using multiplex-PCR for respiratory viruses’ detection. Results A total of 278 patients were enrolled. RSV circulation decreased in the post-nirsevimab season, both in the bronchiolitis and LRTI groups, although it remained the main cause of infections. In the post-nirsevimab season, detections of influenza virus and bocavirus increased in the bronchiolitis cohort (p-values 0.02 and 0.01), while human metapneumovirus and coronaviruses were more frequently identified in children with LRTIs (p-values 0.08 and 0.03). Regarding infants with bronchiolitis, we showed a 59% reduction in hospitalizations rate (both in paediatric ward and paediatric intensive care) ( p -value < 0.001). Children hospitalized for bronchiolitis were older than the previous season (p < 0.001). No differences were observed in the hospitalization rate among infants with viral LRTI. Conclusions Following nirsevimab introduction, circulation of non-RSV respiratory viruses increased, while hospitalizations and ICU admissions for bronchiolitis significantly decreased. Introduction Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis in infants and acute lower respiratory tract infections (LRTIs) in young children worldwide. Nearly all children are infected by RSV at least once by the age of two, with reinfections occurring throughout all the life due to only partial and short-lived immunity [1]. RSV is the most common cause of hospitalization in infants under 12 months of age [2]. RSV infections represent a significant public health challenge because of the considerable burden they place on healthcare systems. It is estimated that RSV is responsible for approximately 33.0 million episodes of acute LRTIs in children under 5 years of age, resulting in around 3.6 million hospital admissions and 101,400 deaths annually [3]. The burden is highest among infants younger than six months, especially in low- and middle-income countries, although substantial morbidity and hospitalizations are also observed in high-income countries [3]. Seasonal peaks of RSV normally lead to overcrowding of paediatric units, increased demand for intensive care resources, and substantial indirect costs related to parental work loss. [4-5]. Until recently, preventive strategies against RSV were limited primarily to non-pharmacological interventions and the use of palivizumab, a monoclonal antibody restricted to high-risk infants. To date, the European Medicines Agency (EMA) and the Italian Medicines Agency (AIFA) have approved new preventive strategies, marking a significant milestone toward immunoprophylaxis and a substantial reduction in the RSV-related burden on healthcare systems. These strategies include maternal vaccination during pregnancy and the administration of a novel antibody, nirsevimab [6]. The latter is a long-acting monoclonal antibody targeting the RSV F protein, offering protection for all RSV season with a single intramuscular dose. It has demonstrated high efficacy in reducing medically attended RSV-associated lower respiratory tract infections (RSV-LRTIs) and hospitalizations in both late preterm and term infants [7-8]. Our national immunization policy required that all newborns had to be immunized with nirsevimab at birth in maternity wards, while, at the same time, infants under 100 days of age should be immunized by their primary care paediatricians [9]. However, in Italy we have twenty regions and as the Italian healthcare system is regionally managed, each region has its own health care strategy and policy. Therefore, organization differences between regions may have influenced the implementation of this strategy. We recall that during the SARS-CoV-2 pandemic, the preventive measures implemented reduced the circulation of viruses, but at the same time after lockdown termination an increase in the circulation of certain bacteria and viruses was observed. This phenomenon was referred to an immunity debt by children for the lockdown [10-12]. Following this, we believe it was of interest to analyse the circulation of viruses in children with LRTIs following the introduction of nirsevimab. The primary objective of this study was to evaluate whether there were changes in the circulation of respiratory viruses among hospitalized children under five years of age with bronchiolitis and viral LRTI in our hospital, after the introduction of Nirsevimab and secondarily, whether the introduction of nirsevimab influenced the hospitalization rate in these two groups. Study design and setting We conducted a retrospective cohort study at the Paediatric Emergency Department of Policlinico Umberto I, a tertiary care university hospital in Rome, from Lazio region in Italy. The study covered two consecutive RSV epidemic seasons: from October 1, 2023 to March 31, 2024 (pre-nirsevimab season), and from October 1, 2024 to March 31, 2025 (post-nirsevimab season). Participants and study size All children under 5 years of age who were hospitalized with a diagnosis of viral LRTI during the study periods were eligible for inclusion (Figure 1). Therefore, no formal sample size calculation was performed. Patients were classified into two mutually exclusive clinical groups: Bronchiolitis group: Infants under 12 months of age hospitalized with bronchiolitis. LRTIs group: Children under 5 years of age admitted for non-bronchiolitis viral LRTIs. In the subgroup of patients with bronchiolitis, we also analysed Emergency Department (ED) attendances and all transfers to the Paediatric Intensive Care Unit (PICU), both directly from the ED and from the Paediatric ward within our institution. Exposure and comparison No randomization or direct measurement of individual immunization status was performed; the exposure is contextual and based on the timing of the immunization program introduction. Bias To avoid misclassification bias regarding exposure to nirsevimab, patients admitted to Paediatric Emergency ward between October 1 and December 14 were excluded from the analysis due to the transitional period following the antibody’s approval. Although nirsevimab was authorized for use in the Lazio region on November 25, 2024 [9], real-world administration was delayed, with the first immunizations occurring at least two weeks later [13]. Ethics Committee Approval and Informed Consent All clinical investigations were conducted according to the Declaration of Helsinki principles. The study was approved by the Ethics Committee of the “Policlinico Umberto I” Hospital (No 2377/2012). The study population belongs to an Italian cohort of children under five years of age hospitalized for LRTIs at the at the Paediatric Emergency Department of Policlinico Umberto I. All persons involved had provided their informed consent prior to inclusion in the study. Data collection For each patient were collected demographic and clinical data. These data were extracted retrospectively from electronic medical records, Galileo ASST of Policlinico Umberto I of Rome. All hospitalized patients at admission underwent nasopharyngeal swab testing using multiplex PCR-QIAstat-Dx for respiratory microbes’ detection (RSV, Influenza virus A/B, Rhinovirus, Human Metapneumovirus A/B, Bocavirus, Parainfluenza Virus 1-2-3-4, Endemics Coronavirus, Adenovirus and B. Pertussis, M. Pneumoniae, C. Pneumoniae). For our analysis, we excluded patients with bacterial infections. Statistical Analysis SPSS 27.0 toolbox (version 27, IBM, New York, USA) was used for statistical analyses. Quantitative variables were treated as continuous variables and reported as mean ± standard deviation. Categorical variables were treated as dichotomous variables and expressed as number and percentage of cases. Comparisons between quantitative variables were done using the Student t-test, and comparisons between qualitative variables were done using the χ2 test. A p-value < 0.05 was considered as statistically significant. Results Patients A total of 278 patients were enrolled across the two epidemic seasons. In the pre-nirsevimab season, 182 patients were included, of whom 119 were diagnosed with bronchiolitis and 62 with viral LTRIs. In the post-nirsevimab season, 97 patients were enrolled, including 49 cases of bronchiolitis and 48 cases of viral LTRIs (Figure 1). Bronchiolitis Despite a 60 % reduction of the number of RSV bronchiolitis during the post-nirsevimab season, RSV remained the primary pathogen responsible for bronchiolitis (Table 1, Figure 2). In contrast, a significant increase in the detection of influenza virus and bocavirus was observed ( p-value 0.02 and p-value 0.01 respectively) (Table 1). In our cohort, most hospitalizations involved non-immunized infants; in fact, only five patients (10 % of bronchiolitis hospitalized during post-nirsevimab season, (mean age of 6.7 months) received nirsevimab prior to admission (Table 1). Among these five patients, RSV was detected as part of a co-infection with another virus in two cases. In the remaining three cases, we identified Influenza virus in one, Rhinovirus in another, and a triple co-infection with Rhinovirus, Human Metapneumovirus, and Coronavirus HKU1 in the last case. A detailed analysis focusing on all patients with bronchiolitis during the two epidemic seasons showed differences in terms of emergency department attendances, admissions to the Paediatric ward and transfers to the PICU (Figure 3). During the post-nirsevimab season, the number of Emergency Department (ED) visits for bronchiolitis decreased from 273 cases (3.1 % of total visits in the ED) in the pre-nirsevimab season to 130 cases (1.6 % of total visits in the ED), representing a 52% reduction ( p-value < 0.001) . Hospitalizations also showed a marked decline, decreasing from 119 in the pre-nirsevimab season to 49 in the post-nirsevimab season, corresponding to a 59% reduction (p-value < 0.001) in the paediatric ward. Similarly, PICU admissions decreased substantially, with only 8 hospitalizations reported compared to 33 during the pre-nirsevimab season, representing a 76% reduction ( p-value < 0.001 ). A monthly analysis of the data from December to March revealed a gradual decline in hospital admissions starting in December, with a peak reduction of up to 75% observed in February (Figure 4). Furthermore, in the post-nirsevimab season, children hospitalized for bronchiolitis were significantly older ( p-value < 0.001 ) and had a higher body weight ( p-value < 0.04 ) compared to those admitted during the pre-nirsevimab season (Table 1 and Figure 5a). During the pre-nirsevimab season, infants aged ≤3 months accounted for 59% of hospitalizations, whereas in the post-nirsevimab season they represented only 43%, showing a significant decrease ( p-value = 0.04 ) (Figure 5b). Acute LRTIs Regarding viral LRTIs in children under 5 years of age, differences were noted in the virus identification. On the contrary, no significant differences were observed between the two epidemic seasons in terms of hospitalization, demographical and clinical variables. A proportional increase was observed in the detection of human metapneumovirus (p-value = 0.08) and other sub-group viruses (coronavirus viruses and adenovirus) (p-value = 0.03) (Table 1, Figure 2). The identification of coronavirus increased from 3 cases (4.8 %) to 7 cases (14.6 %) in post-nirsevimab season. Furthermore, in the post-nirsevimab season, RSV-related LRTI hospitalizations showed a decline, decreasing from 29 cases (47 %) in the pre-nirsevimab season to 15 cases (31 %) in the post-nirsevimab season, corresponding to a 48 % reduction. The proportion of cases in which RSV was identified in co-infection slightly increased (p-value = 0.37) (Table 1). Discussion This study aimed to evaluate the impact of nirsevimab on viruses’ distribution in a cohort of children under five years of age hospitalized for bronchiolitis or other viral LRTIs. We also analysed the hospitalization rate among two epidemic seasons (pre and post-nirsevimab seasons). Our data revealed a 60% reduction in the number of RSV bronchiolitis ED visits, however in both seasons RSV remained the primary pathogen responsible for bronchiolitis. Our finding aligns with Brar et al. [14], who documented that as RSV incidence declined following widespread prophylaxis. Immunization with monoclonal antibodies helps to reduce the number of severe bronchiolitis cases requiring hospitalization. However, by bypassing mucosal immunity, probably it does not prevent the interindividual transmission of RSV. At the same time, we showed a concurrent increase of viral co-infection, in RSV cases. This trend might suggest a potential decrease in RSV virulence, possibly requiring the presence of other viruses to induce bronchiolitis. However, this remains speculative, and further studies would be necessary to confirm or refute these observations. In our cohort, an increased detection of Influenza virus and Bocavirus was observed. Likewise, Suryadevara et al. [15] observed a shift in viral etiology in a U.S. pediatric hospital after both palivizumab and nirsevimab became available, with a proportional rise in influenza and bocavirus detections consistent with our data. The observed rise in influenza-associated bronchiolitis cases among infants, suggests that influenza vaccination for younger infants should be given serious consideration. Furthermore, among patients hospitalized with viral LRTIs, RSV identification decreased. We hypothesize that this shift may be due to reduced RSV circulation also among infants following nirsevimab introduction, potentially limiting circulation among older children. Additionally, a proportional increase was observed in the detection of other viruses, such as human metapneumovirus and viruses classified under sub-group “other virus” (the coronavirus family and adenovirus). The introduction of nirsevimab in our Region resulted in a 52% reduction in emergency department visits, and a 59% reduction in hospital admissions both in the paediatric emergency unit and in the PICU for bronchiolitis. Our results are in strong agreement with randomized trials and real‑world studies. In the MELODY trial, single‑dose nirsevimab conferred a 79 % reduction in hospitalization RSV‑LRTIs during the first RSV season in healthy late‑preterm and term infants [8]. Similarly, the HARMONIE cohort study reported a 70 % decrease in RSV‑related hospitalizations among preterm infants receiving nirsevimab under routine clinical conditions [7]. Finally, our findings are also consistent with the real-world meta-analysis published in The Lancet by Sumsuzzman et al. in 2024, which reported a reduction in RSV-related hospitalizations and intensive care admissions following nirsevimab use [16]. These results are particularly significant as they reflect a substantial reduction in the burden placed on paediatric healthcare facilities. When analysing the monthly reduction in bronchiolitis hospitalization rates, a slow and gradual decline is observed, starting in December and peaking two months later, in February. This trend probably reflects the delayed activation of our healthcare system in implementing infant immunization or pharmacy company supply of the monoclonal antibody. Finally, our result showing older infants with bronchiolitis hospitalized post‑nirsevimab parallels findings by Ernst et al. [17], who reported that passive immunoprophylaxis preferentially protected the youngest, thereby shifting hospitalization demographics toward older infants. In our sample, infants under three months of age constituted most hospital admissions in the pre-nirsevimab season but they were a minority of admissions in the post-nirsevimab period. This can be explained by the fact that our national immunization policy established that all newborns had to be immunized at birth in maternity wards, while infants under 100 days of age were to be immunized by their family paediatricians. The latter strategy was likely less effective, leading to a discrepancy in immunization coverage between newborns (vaccinated at birth) and other infants under 100 days of age who were supposed to receive immunization through primary care paediatricians. A major strength of this study is its observational design, conducted in a single large paediatric hospital representing an Italian region. The study design enabled the evaluation of both bronchiolitis and non-bronchiolitis patients across the pre- and post-nirsevimab periods, contributing to a more comprehensive understanding of viral respiratory infections in young children. However, the study is limited by its retrospective nature and the lack of randomization, which introduces the possibility of residual confounding. Specifically, the lack of individual-level immunization data may have impacted the ability to fully account for variations in exposure to nirsevimab. Furthermore, the exclusion in the analysis of the first’s months of the post-nirsevimab season due to delayed vaccine implementation may limit the generalizability of our findings to the initial weeks of the immunization rollout due to delays in the initiation of the immunization campaign in our region. In conclusiom, nirsevimab has proven effective in reducing severe RSV-related bronchiolitis, significantly easing the burden on healthcare systems during RSV epidemics. Its use should be prioritized for all infants to ensure protection during the first RSV season. However, further research is needed to assess its impact on co-infections and the circulation of other respiratory viruses. While an increase in non-RSV viruses has been observed, hospitalizations and ICU admissions for RSV bronchiolitis have markedly decreased, confirming the important role of this monoclonal antibody in pediatric healthcare. References 1. Hall CB, Weinberg GA, Iwane MK, et al. The burden of respiratory syncytial virus infection in young children. N Engl J Med. 2009 Feb 5;360(6):588-98. doi: 10.1056/NEJMoa0804877.Meissner HC. Viral bronchiolitis in children. N Engl J Med . 2016;374(1):62–72. 2. Meissner HC. Viral Bronchiolitis in Children. N Engl J Med. 2016 Jan 7;374(1):62-72. doi: 10.1056/NEJMra1413456. 3. Li Y, Wang X, Blau DM, et al. Global, regional, and national disease burden estimates of acute lower respiratory infections due to respiratory syncytial virus in children younger than 5 years in 2019: a systematic analysis. Lancet. 2022 May 28;399(10340):2047-2064. doi: 10.1016/S0140-6736(22)00478-0. 4. Virgili F, Nenna R, Di Mattia G, et al. Acute Bronchiolitis: The Less, the Better? Curr Pediatr Rev. 2024;20(3):216-223. doi: 10.2174/0115733963267129230919091338. 5. Matera L, Nenna R, Frassanito A, Petrarca L, Mancino E, Rizzo V, Di Mattia G, Paolo La Regina D, Pierangeli A, Midulla F. Low lymphocyte count: A clinical severity marker in infants with bronchiolitis. Pediatr Pulmonol. 2022 Jul;57(7):1770-1775. 6. Classificazione di medicinali per uso umano ai sensi dell’art. 12 comma 5 del decreto-legge 13 settembre 2012 n. 158 convertito nella legge 8 novembre 2012 n. 189. Agenzia Italiana del Farmaco (AIFA), Determina n.9 del 4.01.2023. 7. Hammitt LL, Dagan R, Yuan Y, et al. Nirsevimab for prevention of RSV in healthy late-preterm and term infants. N Engl J Med . 2022;386(9):837–846. 8. Griffin MP, Yuan Y, Takas T, et al. Single-dose nirsevimab for prevention of RSV in preterm infants. N Engl J Med . 2020;383(5):415–425. 9. Programma di immunizzazione contro il Virus Respiratorio Sinciziale (VRS) in età pediatrica. Intesa Stato-Regioni del 17 ottobre 2024 (Rep. Atti n. 188/CSR). Nuove indicazioni operative. Regione Lazio Registro Ufficiale u.1379124, 08.11.2024. 10. Brañas P, Fontenla F, Castaño-Amores MV, et al. Impact of the SARS-CoV-2 Pandemic on the Epidemiology of Streptococcus pyogenes: A Five-Year Retrospective Study. Microorganisms. 2024 Nov 23;12(12):2403. doi: 10.3390/microorganisms12122403. 11. Nenna R, Pierangeli A, Matera L, Petrarca L, Conti MG, Mancino E, di Mattia G, La Regina DP, Virgili F, Papoff P, Bonci E, Midulla F. Respiratory Syncytial Virus Bronchiolitis Before and After COVID-19 Pandemic: Has the Immunity Debt Been Paid Off? Pediatr Infect Dis J. 2024 Jul 1;43(7):635-639. doi: 10.1097/INF.0000000000004314. Epub 2024 Feb 28. 12. Locht C. Pertussis before, during and after Covid-19. EMBO Mol Med. 2025 Apr;17(4):594-598. doi: 10.1038/s44321-025-00199-2. Epub 2025 Feb 24. 13. Virus Respiratorio Sinciziale: strategia di prevenzione universale della bronchiolite nelle regioni italiane. Società Italiana di Pediatria, Rivista SIP, 03.03.2025 page 10-13. 14. Brar G, Chakraborty T, Malik S, et al. Shifts in viral co‑infection patterns following RSV immunoprophylaxis in infants. Journal of Pediatric Infectious Diseases . 12(2):110–118 (2023). 15. Suryadevara M, Lee A, Prabhu N, et al. Changing epidemiology of pediatric respiratory viruses after monoclonal antibody rollout. Clinical Infectious Diseases . 78(1):45–53 (2024). 16. Sumsuzzman DM, Wang Z, Langley JM, Moghadas SM. Real-world effectiveness of nirsevimab against respiratory syncytial virus disease in infants: a systematic review and meta-analysis. Lancet Child Adolesc Health. 2025 Jun;9(6):393-403. doi: 10.1016/S2352-4642(25)00093-8. Epub 2025 May 1. 17. Ernst C, Bejko D, Gaasch L, et al. Impact of nirsevimab prophylaxis on paediatric respiratory syncytial virus (RSV)-related hospitalisations during the initial 2023/24 season in Luxembourg. Euro Surveill. 2024 Jan;29(4):2400033. doi: 10.2807/1560-7917.ES.2024.29.4.2400033. Figure legends Figure 1. Flowchart of patient enrolment and selection process. Abbreviations: LRTIs: Lower Respiratory Tract Infections. Figure 2. Virus’s detection. A) Numbers of virus detected in patients with bronchiolitis. B) Percentage of virus detected in patients with bronchiolitis. C) Numbers of virus detected in patients with viral LRTIs. D) Percentage of virus detected in patients with viral LRTIs. Figure 3. Bronchiolitis-related emergency department attendances, ward admissions, transfers to the PICU, and need for mechanical ventilation at our institution. Abbreviations: ED: Emergency Department. PEU: Pediatric Emergency Unit. PICU: Pediatric Intensive Care Unit Figure 4. Monthly trend of bronchiolitis hospitalizations at our institution during the 2023–2025 epidemic seasons. Abbreviations: PEU: Pediatric Emergency Unit. Figure 5a. Monthly trend of age among patients hospitalized for bronchiolitis. ’All’ refers to the entire post-nirsevimab period. Figure 5b. Comparison of the two epidemic seasons regarding the number of hospital admissions for bronchiolitis in infants under and over 3 months of age. Supplementary Material File (figures.docx) Download 315.30 KB File (table 1.docx) Download 19.68 KB Information & Authors Information Version history V1 Version 1 09 June 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords bronchiolitis lrtis nirsevimab virus Authors Affiliations Domenico Paolo La Regina 0000-0002-8297-8920 Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author M. Spatuzzo Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author Enea Bonci Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author Raffaella Nenna 0000-0001-8880-3462 Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author Laura Petraca 0000-0002-2430-2981 Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author Maria Giulia Conti Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author Enrica Mancino 0000-0003-2756-0094 Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author Alessandra Pierangeli 0000-0003-0633-360X Sapienza University of Rome View all articles by this author Guido Antonelli 0000-0002-2533-2939 Sapienza University of Rome View all articles by this author Fabio Midulla 0000-0001-7476-5266 [email protected] Universita degli Studi di Roma La Sapienza Dipartimento Materno Infantile e Scienze Urologiche View all articles by this author Metrics & Citations Metrics Article Usage 315 views 215 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Domenico Paolo La Regina, M. Spatuzzo, Enea Bonci, et al. Virus detection in infants with bronchiolitis and children with Lower Respiratory Tract Infections after nirsevimab introduction: a retrospective comparative two-season study. Authorea . 09 June 2025. DOI: https://doi.org/10.22541/au.174943635.57904021/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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