Respiratory syncytial virus (RSV) has long been recognized as a primary pathogen responsible for acute respiratory infections in young children worldwide. Recent comprehensive research conducted by Karolinska Institutet in Sweden has shed new light on the epidemiology and risk factors associated with severe RSV outcomes, fundamentally challenging previous clinical assumptions. Published in The Lancet Regional Health – Europe, this large-scale observational cohort study analyzed data from over 2.3 million children born in Sweden between 2001 and 2022, exploring the incidence and severity of RSV infections and the characteristics of affected populations.
RSV traditionally has been viewed as a significant threat mainly to preterm infants and children with pre-existing chronic conditions such as congenital heart disease or immunodeficiencies. However, this landmark study reveals a striking—and somewhat alarming—reality: even healthy, full-term infants without any known comorbidities are at substantial risk of developing severe RSV infections that necessitate intensive care or result in extended hospital stays, particularly within the critical first three months following birth. This overturns the conventional wisdom that primarily vulnerable groups dominate the severe case statistics.
The researchers found that among children hospitalized due to RSV, the largest subgroup requiring intensive care or prolonged hospitalization were previously healthy, full-term infants younger than three months old. This subset accounted for a significant proportion of severe RSV cases, highlighting the vulnerability of even ostensibly low-risk populations during early infancy. Such data underscore the critical need for preventive measures that encompass a broader demographic than previously targeted in RSV prophylaxis strategies.
Central to addressing this public health challenge is the advent of new preventive therapies. Historically, prophylaxis with monoclonal antibodies has been limited to high-risk infants, notably preterm babies and those with chronic illnesses. However, the newly approved monoclonal antibody, slated for implementation in Sweden starting in September 2025, offers a broader protective reach. This passive immunization, administered to all newborns during the RSV season, mimics vaccination by providing sustained protection for approximately six months, aligning coverage with the period of highest vulnerability.
In addition to passive immunoprophylaxis for newborns, developments in maternal vaccination strategies offer another promising avenue for disease prevention. Vaccinating pregnant women to boost transplacental antibody transfer to the fetus could confer passive immunity immediately after birth, filling the immunity gap before infants are eligible for immunization themselves. Such integrated approaches could revolutionize RSV control, reducing the burden of severe respiratory illnesses in infants at a population level.
The Swedish cohort study further revealed epidemiological factors that increase the risk of severe RSV outcomes. Infants born during the winter months demonstrated approximately a threefold greater risk of severe disease compared to those born during other seasons, likely reflecting seasonal viral circulation dynamics. Moreover, the presence of siblings aged 0 to 3 years or being a twin increased the risk of intensive care admission, which may be associated with heightened viral transmission within households.
Birthweight also emerged as a critical determinant of RSV severity. Infants who were small for gestational age exhibited nearly four times the risk of experiencing serious disease manifestations. This finding suggests that intrauterine growth restriction and possibly associated physiological immaturity could impair an infant’s resilience against viral respiratory infections. Underlying medical conditions conferred the highest risk increase, with a more than fourfold elevated likelihood of severe disease or mortality, reinforcing the importance of targeted protection in this subgroup.
Perhaps most concerning is the median age of infants requiring intensive care, established as just under two months. At this early developmental stage, the immune system remains immature, and pulmonary structures are still developing, rendering infants particularly vulnerable to respiratory pathogens like RSV. Clinical practice must therefore prioritize protection within this narrow window, supporting recent changes in prophylaxis policy expanding eligibility criteria.
Importantly, this Swedish research aligns with and expands upon findings from other Nordic and European investigations, contributing robust data with national registry-level completeness. The observational study design, while not interventional, provides high external validity and the statistical power necessary to detect risk associations across large populations and multiple variables.
From a public health perspective, the implications of this research are profound. Broadened prophylactic coverage has the potential to reduce RSV-associated morbidity dramatically and alleviate the strain on pediatric intensive care units during seasonal peaks. The adoption of monoclonal antibody prophylaxis for all newborns represents a paradigm shift, fundamentally changing approaches to RSV prevention and control.
As researchers and clinicians prepare to implement these new guidelines, continued surveillance and real-world effectiveness studies will be essential to monitor impacts on hospitalization rates, disease severity, and health system burdens. Moreover, ongoing development of vaccines for pediatric use promises to add another tool for RSV control in coming years.
This study, carried out collaboratively with partners at the University of Helsinki and Helsinki University Hospital in Finland, was supported by multiple Swedish funding bodies, including the Swedish Research Council and the Swedish Heart-Lung Foundation. While some researchers have disclosed unrelated consulting engagements with pharmaceutical companies, no conflicts bear upon the present findings.
In sum, RSV remains a formidable threat, not only to traditionally recognized high-risk infants but also to previously healthy newborns, especially those under three months of age. With advances in immunoprophylaxis and vaccination on the horizon, the opportunity exists to significantly change the clinical trajectory of RSV infections, reduce severe childhood respiratory illness, and improve long-term pediatric health outcomes globally.
Subject of Research: People
Article Title: Risk factors for severe outcomes of respiratory syncytial virus infection in children: a nationwide cohort study in Sweden
News Publication Date: 10-Sep-2025
Web References:
https://www.thelancet.com/journals/lanepe/article/PIIS2666-7762(25)00239-X/fulltext
References:
Dallagiacoma G, Lundholm C, Smew AI, et al. Risk factors for severe outcomes of respiratory syncytial virus infection in children: a nationwide cohort study in Sweden. The Lancet Regional Health – Europe. 2025; doi:10.1016/j.lanepe.2025.101447.
Keywords: Pediatrics, Respiratory syncytial virus, Viral infections, Vaccine research, Drug studies, Infectious diseases, Children, Obstetrics, Neonatology
