Objective To determine whether any association exists between exposure to 2009 pandemic H1N1 (pH1N1) influenza vaccination during pregnancy and negative health outcomes in early childhood.

Design Retrospective cohort study.

Setting Population based birth registry linked with health administrative databases in the province of Ontario, Canada.

Participants All live births from November 2009 through October 2010 (n=104 249) were included, and children were followed until 5 years of age to ascertain study outcomes.

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Credit: iStock, JovanmandicCite this as: BMJ 2019;366:l4151    http://dx.doi.org/10.1136/bmj.l4151