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Abstract Respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory infections in children. By the age of 1 year, 60%–70% of children have been infected by RSV. In addition, early-life RSV infection is associated with the development of recurrent wheezing and asthma in infancy and childhood. The need for precise epidemiologic data regarding RSV as a worldwide pathogen has been growing steadily as novel RSV therapeutics are reaching the final stages of development. To optimize the prevention, diagnosis, and treatment of RSV infection in a timely manner, knowledge about the differences in the timing of the RSV epidemics worldwide is needed. Previous analyses, based on literature reviews of individual reports obtained from medical databases, have failed to provide global country seasonality patterns. Until recently, only certain countries have been recording RSV incidence through their own surveillance systems. This analysis was based on national RSV surveillance reports and medical databases from 27 countries worldwide. This is the first study to use original-source, high-quality surveillance data to establish a global, robust, and homogeneous report on global country-specific RSV seasonality. Respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory infections in children. This is the first study to use original-source, high-quality surveillance data to establish a global, robust, and homogeneous report on global country-specific RSV seasonality.

Respiratory syncytial virus (RSV) is the leading global cause of respiratory infections in infants and the second most frequent cause of death during the first year of life. This highly contagious seasonal virus is responsible for approximately 3 million hospitalizations and 120,000 deaths annually among children under the age of 5 years. Bronchiolitis is the most common severe manifestation; however, RSV infections are associated with an increased long-term risk for recurring wheezing and the development of asthma. There is an unmet need for new agents and a universal strategy to prevent RSV infections starting at the time of birth. RSV is active between November and April in Italy, and prevention strategies must ensure that all neonates and infants under 1 year of age are protected during the endemic season, regardless of gestational age at birth and timing of birth relative to the epidemic season. Approaches under development include maternal vaccines to protect neonates during their first months, monoclonal antibodies to provide immediate protection lasting up to 5 months, and pediatric vaccines for longer-lasting protection. Meanwhile, improvements are needed in infection surveillance and reporting to improve case identification and better characterize seasonal trends in infections along the Italian peninsula. Rapid diagnostic tests and confirmatory laboratory testing should be used for the differential diagnosis of respiratory pathogens in children. Stakeholders and policymakers must develop access pathways once new agents are available to reduce the burden of infections and hospitalizations.

Respiratory Syncytial Virus (RSV) is a leading cause of morbidity and hospitalization in all infants. Many RSV vaccines and monoclonal antibodies (mAb) are currently under development to protect all infants, but to date preventive options are available only for preterms. In this study, we assessed the knowledge, attitudes, and practices towards RSV and the preventive use of mAb in a sample of Italian Pediatricians. An internet survey was administered through an internet discussion group, with a response rate of 4.4% over the potential respondents (No. 389 out of 8842, mean age 40.1 ± 9.1 years). The association of individual factors, knowledge, and risk perception status with the attitude towards mAb was initially inquired by means of a chi squared test, and all variables associated with mAb with p < 0.05 were included in a multivariable model calculating correspondent adjusted Odds Ratio (aOR) with 95% confidence intervals (95%CI). Of the participants, 41.9% had managed RSV cases in the previous 5 years, 34.4% had diagnosed RSV cases, and 32.6% required a subsequent hospitalization. However, only 14.4% had previously required mAb as immunoprophylaxis for RSV. Knowledge status was substantially inappropriate (actual estimate 54.0% ± 14.2, potential range 0–100), while the majority of participants acknowledged RSV as a substantial health threat for all infants (84.8%). In multivariable analysis, all these factors were characterized as positive effectors for having prescribed mAb (aOR 6.560, 95%CI 2.904–14.822 for higher knowledge score; aOR 6.579, 95%CI 2.919–14.827 for having a hospital background, and a OR 13.440, 95%CI 3.989; 45.287 for living in Italian Major Islands). In other words, reporting less knowledge gaps, having worked in settings with a higher risk of interaction with more severe cases, and being from Italian Major Islands, were identified as positive effectors for a higher reliance on mAb. However, the significant extent of knowledge gaps highlights the importance of appropriate medical education on RSV, its potential health consequences, and the investigational preventive interventions.

Background Respiratory syncytial virus (RSV) is a leading cause of acute respiratory infections worldwide. While historically RSV research has been focused on children, data on RSV infection in adults are limited. The goal of this study was to establish the prevalence of RSV in community-dwelling Italian adults and analyze its genetic variability during the 2021/22 winter season. Methods In this cross-sectional study, a random sample of naso-/oropharyngeal specimens from symptomatic adults seeking for SARS-CoV-2 molecular testing between December 2021 and March 2022 were tested for RSV and other respiratory pathogens by means of reverse-transcription polymerase chain reaction. RSV-positive samples were further molecularly characterized by sequence analysis. Results Of 1,213 samples tested, 1.6% (95% CI: 0.9–2.4%) were positive for RSV and subgroups A (44.4%) and B (55.6%) were identified in similar proportions. The epidemic peak occurred in December 2021, when the RSV prevalence was as high as 4.6% (95% CI: 2.2–8.3%). The prevalence of RSV detection was similar ( p  = 0.64) to that of influenza virus (1.9%). All RSV A and B strains belonged to the ON1 and BA genotypes, respectively. Most (72.2%) RSV-positive samples were also positive for other pathogens being SARS-CoV-2, Streptococcus pneumoniae and rhinovirus the most frequent. RSV load was significantly higher among mono-detections than co-detections. Conclusion During the 2021/22 winter season, characterized by the predominant circulation of SARS-CoV-2 and some non-pharmaceutical containment measures still in place, a substantial proportion of Italian adults tested positive for genetically diversified strains of both RSV subtypes. In view of the upcoming registration of vaccines, establishment of the National RSV surveillance system is urgently needed.

Objective Respiratory syncytial virus (RSV) is a leading cause of severe lower respiratory infection, and therefore, a major threat to global health. This study determined the epidemiological and molecular characteristics of RSV among cases of influenza‐like illness (ILI) and severe acute respiratory infection (SARI) among children in the Philippines. Method The study included archived nasopharyngeal swab and oropharyngeal swab samples collected from patients under the age of five who are presented with ILI or SARI for the period of 2006–2016. Swabs were examined for RSV subgroup by multiplex real‐time qRT‐PCR. Partial genome sequencing and phylogenetic analyses of the second hypervariable region (HVR) of the G gene were used to determine the genotype of RSV isolates. Results A total of 1036 representative samples from all sites were selected and tested. Of these samples, 122 were RSV‐positive at 11.8% prevalence rate, and 58.2% (71/122) were classified as RSV‐A. Six genotypes were identified, which include NA1 (27/122, 22.1%), ON1 (5/122, 4.1%), GA2 (1/122, 0.8%), and GA5 (1/122, 0.8%) for RSV‐A; and BA2 (13/122, 10.7%) and BA9 (1/122, 0.8%) for RSV‐B. Most RSV‐related cases were significantly associated with clinical characteristics such as runny nose (88.1% RSV vs. 11.9% non‐RSV: p value = 0.021), pneumonia (80.6% RSV vs. 19.4% non‐RSV; p value = 0.015), and bronchitis (71.7% RSV vs. 28.3% non‐RSV; p value < 0.001). Increased RSV‐related cases were observed among children below 24 months old. Conclusion The RSV trend and genetic variability in the Philippines resembles a similar pattern of transmission globally.

Background RSV-incidence estimates obtained from routinely-collected healthcare data (e.g., MarketScan) are commonly adjusted for under-reporting using test positivity reported in national Surveillance Systems (NREVSS). However, NREVSS lacks detail on patient-level characteristics and the validity of applying a single positivity estimate across diverse patient groups is uncertain. We aimed to describe testing practices and test positivity across subgroups of private health insurance enrollees in the US and illustrate the possible magnitude of misclassification when using NREVSS to correct for RSV under ascertainment. Methods Using billing records, we determined distributions of RSV-test claims and test positivity among a national sample of private insurance enrollees. Tests were considered positive if they coincided with an RSV-diagnosis. We illustrated the influence of positivity variation across sub-populations when accounting for untested acute respiratory infections. Results Most tests were for children (age 0–4: 65.8%) and outpatient encounters (78.3%). Test positivity varied across age (0–4: 19.8%, 5–17: 1.8%, adults: 0.7%), regions (7.6–16.1%), settings (inpatient 4.7%, outpatient 14.2%), and test indication (5.0–35.9%). When compared to age, setting or indication-specific positivity, bias due to using NREVSS positivity to correct for untested ARIs ranged from − 76% to 3556%. Conclusions RSV-test positivity depends on the characteristics of patients for whom those tests were ordered. NREVSS-based correction for RSV-under-ascertainment underestimates the true incidence among children and overestimate rates among adults. Demographic-specific detail on testing practice and positivity can improve the accuracy of RSV-incidence estimates.

•A preventive RSV vaccine for use in infants is an important unmet medical need.•Past studies observed vaccine-associated ERD after RSV infection in RSV-naïve infants.•Risk of vaccine-associated ERD has stalled efforts to develop new candidate vaccines.•Understanding the immunopathology of ERD has guided renewed efforts at vaccine development.•This report summarizes presentations and discussion of a recent VRBPAC meeting. Respiratory syncytial virus (RSV), is a common cause of serious acute lower respiratory tract illness in infants and young children, causing substantial morbidity and mortality globally. Treatment is mainly supportive and currently there is no licensed preventive vaccine. Clinical trials conducted in the 1960s evaluating a formalin-inactivated RSV vaccine (FI-RSV) in RSV-naïve infants resulted in observations of enhanced respiratory disease (ERD) following subsequent natural RSV infection in vaccinees. In these studies, infants immunized with FI-RSV had higher rates of severe RSV disease compared with controls. This outcome redirected focus on identifying the immunologic mechanisms that precipitated ERD as a prerequisite to further vaccine development. Improved understanding of the immunopathogenesis of ERD derived from animal models has stimulated development of new candidate vaccines and engendered discussions among RSV experts about the safety data needed to advance these products into the clinic, and ultimately, into the target population of RSV-naïve infants. The recognition that multiple products would soon be ready for testing in infants and children prompted the FDA to hold a Vaccines and Related Biological Products Advisory Committee (VRBPAC) meeting to seek perspectives and advice of experts regarding the types and extent of preclinical and clinical data that might be needed to support testing in RSV-naïve infants for specific types of candidate RSV vaccines. Committee members agreed that, if certain conditions are met in preclinical and early clinical studies, it would be reasonable to move forward from studies in adults and older children and into clinical trials evaluating vaccine safety and efficacy in RSV-naïve infants. Herein, we review and summarize perspectives on the discussion regarding recommendations for RSV vaccine development in this population.

Background. In the United States, the seasonality of respiratory syncytial virus (RSV) has traditionally been defined on the basis of weeks during which antigen-based tests detect RSV in >10% of specimens (hereafter, the \"10% threshold\"). Because molecular testing has become more widely used, we explored the extent of polymerase chain reaction (PCR)–based RSV testing and its impact on determining the seasonality of RSV. Methods. We assessed antigen- and PCR-based RSV reports submitted to the National Respiratory and Enteric Virus Surveillance System during July 2005–June 2015. To characterize RSV seasons by using PCR-based reports, we assessed the traditional 10% threshold; subsequently, we developed 3 methods based on either PCR-based detections or the percentage of positive test results. Results. The annual number of PCR-based reports increased 200-fold during 2005–2015, while the annual number of antigen-based reports declined. The weekly percentage of specimens positive for PSV by PCR was less than that for antigen-detection tests; accordingly, the 10% threshold excluded detections by PCR and so was imprecise for characterizing RSV seasons. Among our PCR-specific approaches, the most sensitive and consistent method captured 96%–98% of annual detections within a season, compared with 82%–94% captured using the traditional method. Conclusions. PCR-based reports are increasingly relevant for RSV surveillance and determining the seasonality of RSV. These PCR-specific methods provide a more comprehensive understanding of RSV trends, particularly in settings where testing and reporting are most active. Diagnostic practices will vary by locality and should be understood before choosing which method to apply.

We previously demonstrated that vaccination with RSV-Mnull, a prototype single-cycle live vaccine lacking the matrix (M) gene, generated anti-viral serum IgG and memory T cell responses, and reduced challenge virus shedding and pulmonary dysfunction in mice. Here we further characterized the response to RSV-Mnull, and designed and tested second generation Mnull vaccines. In mice, prime-boost vaccination with RSV-Mnull generated pre-fusion (preF) and attachment protein (G) -specific serum IgG and lung IgA, and protected from lung pathology, showing that a single-cycle live vaccine was effective in this model. In an effort to enhance efficacy for future human application, second generation Mnull vaccines were designed, in which nonstructural protein 1 (NS1), a known interferon (IFN) antagonist, was relocated to reduce expression. In addition, the G or F genes were moved to the first genome position (RSV-Mnull/G1 and RSV-Mnull/F1 respectively). In vitro, RSV-Mnull/G1 and RSV-Mnull/F1 showed reduced NS1 levels and increased IFN-β induction, whereas IFN-λ levels were not affected. Viruses with relocated NS1 also displayed enhanced anti-viral state in uninfected cells. RSV-Mnull/G1 induced higher levels of anti-G serum antibodies (Abs), whereas RSV-Mnull/F1 increased the ratio of anti-preF:anti-G IgG Abs. RSV-Mnull induced lower levels of lung IgA than wildtype RSV; However, relocation of NS1 increased early IgA induction and restored IgA levels to those seen with wildtype RSV. These findings suggest differences in G and F presentation or processing and Ab induction, and indicate that the genomic location of NS1, G, and F can impact/improve IgG and IgA levels and timing. All vaccines induced Abs that neutralized RSV in vitro and protected animals against a high challenge dose of wildtype RSV. In terms of protection against lung pathology, we did not see major improvements of RSV-Mnull/G1 and RSV-Mnull/F1 over the RSV-Mnull prototype, potentially due to limitations of the model. Nevertheless, RSV-Mnull/G1 and RSV-Mnull/F1 elicited high Ab levels against both major antigens, enhanced serum anti-G IgG or lung anti-F IgA levels, and protected mice from lung pathology in spite of single-cycle replication. Thus, the single-cycle approach has potential to be a platform for development of safe and efficacious live RSV vaccines.

Clesrovimab (MK-1654) is a next-generation monoclonal antibody (mAb) engineered for the prophylaxis of Respiratory Syncytial Virus (RSV or HRSV), a major etiological agent of lower respiratory tract infections in newborns, the elderly, and immunocompromised individuals. Having received regulatory approval in the United States and the United Arab Emirates in 2025, and currently under review in additional global markets, Clesrovimab has demonstrated robust immunoprophylactic efficacy in clinical trials. A single-dose administration provides durable protection, a significant reduction in RSV-associated infections and hospitalizations versus placebo, and a favorable safety profile with a lower incidence of adverse events (AEs) compared with existing standard-of-care interventions. This narrative review summarizes Clesrovimab’s molecular characteristics and clinical development, outlining key improvements observed in late-phase trials. Relevant data were identified through a literature search of PubMed, Scopus, and ClinicalTrials.gov (2018–2025) using the keywords “Clesrovimab,” “MK-1654,” and “respiratory syncytial virus”. A comparative analysis is presented with currently approved RSV-targeting mAbs, such as Palivizumab and Nirsevimab. The aim is to highlight Clesrovimab’s potential as a novel preventive strategy against RSV infection, emphasizing its enhanced binding affinity for the viral fusion (F) protein, superior biodistribution within the respiratory tract, and capacity to provide more effective and sustained protection. These features support its promising role in reducing RSV-related morbidity and mortality among high-risk populations.