Human respiratory syncytial virus (RSV) causes severe disease in young infants. Besides young age, prematurity and congenital cardiopulmonary disease are known risk factors for severe disease. However, most children admitted to the hospital with RSV are previously healthy. To identify those at risk for severe disease, biomarkers are needed. In the RESCEU case-control study we established biomarkers predictive of, or associated with, lower respiratory tract RSV infection and disease severity. In this follow-up study the detected biomarkers will be validated.