Timing of birth as an emergent risk factor for rotavirus hospitalization and vaccine performance in the post-vaccination era in the United States.

Rotavirus vaccines were introduced in the United States in 2006 and in the subsequent years have fundamentally altered seasonality and shifted disease from annual to biennial epidemics. We investigated whether season and year of birth have emerged as risk factors for rotavirus and affected vaccine performance. We constructed a retrospective birth cohort of US children <5 years using the 2001-2014 MarketScan Database. We evaluated the assocations of season of birth, even/odd year of birth and interactions with vaccination. We fit Cox proportional hazards models to estimate the hazard of rotavirus hospitalization by calendar year of birth, season of birth assessed for interaction with vaccination. After vaccine introduction, we observed monotonically decreasing rates of rotavirus hospitalization for each subsequent birth cohort, but a biennial incidence pattern by calendar year. In the post-vaccine period, children born odd calendar years had higher hazard of rotavirus hospitalization than even year births. Children born in winter had the highest hazard of hospitalization but also had the higher vaccine effectiveness than spring, summer or fall births. With the emergence of a strong biennial pattern of disease following vaccine introduction, the timing of a child's birth has become a risk factor for rotavirus.