Lopman B, et al. - A retrospective birth cohort of US children <5 years of age was assembled using the 2001–2014 MarketScan Database to determine if season and year of birth are risk factors for rotavirus and vaccine performance. Children born odd calendar years had a higher HR for rotavirus hospitalization than even year births in the post-vaccine period. The highest HR for hospitalization, along with the higher vaccine effectiveness, was observed among children born in winter than spring, summer, or fall births. The timing of a child's birth appeared to be a risk factor for rotavirus with the emergence of a strong biennial pattern of disease following vaccine introduction.