We aimed to study respiratory syncytial virus (RSV) genotype distribution, clinical presentation, and disease severity in infants with bronchiolitis from RSV subtypes and new RSV genotypes. Methods: We prospectively enrolled previously healthy term infants less than one year old hospitalized for bronchiolitis in an Italian University hospital over 12 epidemic seasons. In 312 nasopharyngeal washings positive to RSV, we sequenced the viral genotype and analysed it according to patient data. Strain-specific RSV loads were quantified for 300 specimens. Results: Up to 2011-2012, the RSV-A genotype NA1 predominated, replaced in 2012 by the novel ON1. All infants infected by RSV subtype B were genotype BA. Stratifying bronchiolitis in NA1, ON1 and BA showed that NA1-infected infants were the youngest and had the most severe clinical course. Conversely, BA-infected infants had less severe symptoms and more frequently had eosinophilia and a family history for asthma. Infants with the ON1 genotype, had a milder clinical course than those with NA1 and more risk factors for asthma, despite having the highest viral loads. Conclusion: The disease course in infants hospitalized for acute RSV bronchiolitis may depend on the RSV genotype.
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