Season and Year of Birth an Emergent Risk Factor for Rotavirus

Researchers found being born in an odd calendar year, especially in the winter or fall seasons, emerged as a risk factor for rotavirus in the postvaccine era in the United States.

Since the introduction of rotavirus vaccines to the United States in 2006, a strong biennial pattern of disease has emerged, leading to the timing of birth becoming a risk factor for rotavirus, according to research in the American Journal of Epidemiology.

Investigation into season and year of birth as rotavirus risk factors was performed on a retrospective birth cohort of US children <5 years of age using the 2001 through 2014 MarketScan database. Cox proportional hazards models estimated the hazard of rotavirus hospitalization by calendar year of birth and season of birth.

Prior to vaccine introduction, the rates of hospitalization did not vary by year of birth, with cases ranging from 16.1 to 20.3 per 10,000 person-years. Rates dropped after introduction of the vaccine and stabilized in 2010 with a range of 1.0 to 1.6 per 10,000 person-years.

In the postvaccine period, children born in spring (hazard ratio [HR] 0.68; 95% CI, 0.57-0.82), summer (HR 0.59; 95% CI, 0.48-0.72), and fall (HR 0.81; 95% CI, 0.66-0.99) had lower hazard of hospitalization compared with those born during the winter after controlling for odd or even calendar year of birth. Following vaccination introduction, children born in odd calendar years had significantly increased hazards than those born in even calendar years (HR 1.35; 95% CI, 1.16-1.57).

Vaccine effectiveness (VE) in this study was similar to that of previous estimates using this dataset and others. There was, however, evidence that 2-dose efficiency was lower for children born in spring (VE 84%; 95% CI, 75-89; P =.016), summer (VE 89%; 95% CI, 81-96; P =.041), or fall (VE 88%; 95% CI, 78-94; P =.015) compared with those born in the winter (VE 96%; 95% CI, 91-98).

Investigators noted several limitations with the MarketScan database. It did not include patients on Medicaid. Moreover, teasing apart the effects of birth season, vaccination season, and age-specific risks is difficult because these factors themselves are correlated. However, given the size of the cohort and independence of measurement of birth timing, vaccination, and disease outcome, the database is still a good source to investigate these questions. Finally, differences found regarding younger age distributions in high incidence years could be influenced by biased testing of infants.

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The results suggest that vaccination has brought about a new pattern of disease in the United States and investigators note it is likely that, “the postvaccination dynamics have not yet stabilized and the biennial pattern may continue to evolve.” They recommend that increased importance of vaccinating children be placed on “children born in advance of a relatively large forthcoming season.”


Lopman B, Dahl R, Shah M, Parashar UD. Timing of birth as an emergent risk factor for rotavirus hospitalization and vaccine performance in the post-vaccination era in the United States [published online March 13, 2018]. Am J Epidemiol. doi: 10.1093/aje/kwy054