This observational cohort study aims to estimate the rate of transmission of vaccine-strain rotavirus from vaccinated to unvaccinated infants during a one-year period in a U.S. NICU that routinely administers rotavirus vaccine to a large number of infants (>180 annually) who usually remain in the unit for a long period after receipt. The purpose of this study is to show how rotavirus vaccination can be given safely in the NICU setting.