TOPLINE:

In a nationwide cohort of over 2.3 million Swedish children, winter birth, small for gestational age, multiple birth, and preterm birth increased the likelihood of respiratory syncytial virus (RSV)-related severe outcomes, such as ICU admission, death, or prolonged hospitalisation.

METHODOLOGY:

• Researchers in Sweden conducted a nationwide register-based cohort study to identify risk factors associated with severe outcomes of RSV infection among 2,354,302 children (51.4% boys) born between January 2001 and December 2022.
• Information regarding sociodemographic characteristics and comorbidities was obtained from linked registers; the diagnosis of RSV infection and its related hospitalisations and deaths were identified via diagnostic codes.
• Outcomes were RSV-associated ICU admission or death and prolonged hospitalisation, defined as a hospital stay of at least 7 days.
• Children were followed up from day 8 of life until the occurrence of the outcome or until censoring, with a minimum follow-up duration of 15 months.

TAKEAWAY:

• Overall, 38,919 children were diagnosed with RSV infection, of whom 1210 required ICU admission, 27 died, and 3766 had prolonged hospitalisation.
• Risk factors for RSV-associated ICU admissions or deaths included winter birth (adjusted hazard ratio [aHR], 2.96; 95% CI, 2.53-3.46), being born small for gestational age (aHR, 3.91; 95% CI, 3.08-4.97), having siblings aged 0-3 years (aHR, 2.92; 95% CI, 2.57-3.31), and multiple birth (aHR, 3.43; 95% CI, 2.80-4.21).
• Risk factors for prolonged hospitalisation were similar to those for ICU admissions or deaths, with preterm birth being an additional risk factor.
• Comorbidities such as severe congenital heart disease and Down syndrome conferred a higher risk for prolonged hospital stays.

IN PRACTICE:

"We identified additional risk factors, including being born small for gestational age, multiple birth, and having siblings aged 0-3 years, that are not currently included in RSV immunization policies but may confer increased vulnerability to severe outcomes of RSV infection," the authors wrote.

"A comprehensive immunization strategy that broadens eligibility criteria to include these factors, combined with public health intervention targeting parents, may be essential to more effectively protect the most vulnerable infants," they added.

SOURCE:

This study was led by Giulia Dallagiacoma, Karolinska Institutet, Stockholm, Sweden. It was published online on September 09, 2025, in The Lancet Regional Health - Europe.

LIMITATIONS:

Diagnostic practices for RSV infection varied by region and over time, with testing notably inconsistent in older children and those with milder illnesses. Additionally, this study lacked clinical data, limiting the characterisation of disease severity and ICU admissions before 2008.

DISCLOSURES:

This study was financially supported by KID funding fromKarolinska Institutet, the Swedish Research Council, the Swedish Heart Lung Foundation, the Swedish Asthma and Allergy Association Research Fund, and other organisations. Some authors reported receivingresearch and travel grants, as well as consultancy and lecture fees, and having other ties with multiple pharmaceutical companies and organisations. One author reported being an employee of Finnish Vaccine Research, which conducts vaccine studies funded by several vaccine manufacturers.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.