Respiratory syncytial virus (RSV) infection is the most common cause of severe lower respiratory tract infection (LRTI) in the pediatric population worldwide. Age at the time of infection, prematurity, multiparity, exposure to smoke and the level of passive immunity transmitted at birth are the main risk factors for lower respiratory infection associated with RSV. Other factors, including the innate immune response, respiratory microbiota, and intra-host viral heterogeneity, may also affect outcomes but are not fully considered in RSV infection. Exploring the impact of these factors is difficult due to the heterogeneity of the population which makes statistical adjustment difficult. Thus, twin models are useful in understanding the impact of the host on the environment, as twins often share similar exposure to infection and many risk factors, but not all are ie different prenatal and postnatal conditions, differential transfer of maternal antibodies and the genetic makeup of heterozygotes.

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

Twins identified from the birth cohort (Hospices Civils de Lyon) with at least one twin hospitalized for an acute RSV-infection