Background. Because the production of an effective respiratory syncytial virus (RSV) vaccine for infants is challenging, vaccination of other family members is one viable alternative to prevent severe RSV illnesses in infants.

Methods. In a prospective study, we enrolled all family members of children who were hospitalized with RSV infection. Nasal swabs for RSV detection were obtained from all participating family members. Data on respiratory symptoms in the family members prior to and after the child's admission were collected using standardized questionnaires.

Results. At the time of or within 1 week after the index child's hospitalization, RSV was detected in 40 (77%) of the 52 families and in 60 (47%) of 129 family members. Forty-nine (82%) of RSV detections in the family members were associated with respiratory symptoms. A sibling or a parent was the probable primary case of RSV in 30 (58%) families. Respiratory syncytial virus loads in the nasal swabs were significantly higher (10^7.7) in index children than in their parents (10^5.1, P < .0001).

Conclusions. In most cases, the likely source of an infant's RSV infection is an older sibling or a parent. These findings support the strategy of reducing the burden of RSV in infants by vaccination of their family members.