Inclusion Criteria:  Twins born at Hospices Civils de Lyon in the years 2012 to 2020 At least one twin hospitalized with positive RSV-PCR during the first year of life  Exclusion Criteria:  Major differential underlying disease between twins Death of a twin before the RSV epidemic period of the first year of life Nosocomial infection Lack of understanding of the study or questionnaire by parents Lack of parental consent