pca Yamada 2012.

Methods	Questionnaire‐based, retrospective cohort study performed at the 121 obstetrical facilities of Hokkaido (Japan)
Participants	All 121 obstetric facilities in Hokkaido were requested to deliver a 12‐item questionnaire to all postpartum women who gave birth between 1 December 2009 and 31 May 2010 during their stay in obstetric facilities. About 1/3 of the women who delivered in Hokkaido during this time answered the questionnaire (n = 7535).
Interventions	Influenza vaccination during pregnancy. Out of the 7535 women who answered the questionnaire, 4921 received pandemic influenza vaccine. Among them, 2212 were also reported to have been vaccinated with seasonal vaccine. A further 270 (considered as unvaccinated) received seasonal vaccine only.
Outcomes	Influenza. Definition was not provided. All information was collected by means of a questionnaire, on which items about admission to the intensive care unit, intubation or ventilation, and diagnosis of influenza encephalopathy were also present.
Notes	Strongly biased Government funded
Risk of bias
Bias	Authors' judgement	Support for judgement
PCS/RCS ‐ selection exposed cohort All outcomes	High risk	By interview
PCS/RCS ‐ selection non‐exposed cohort All outcomes	High risk	By interview
PCS/RCS ‐ comparability All outcomes	High risk	No matching
PCS/RCS ‐ assessment of outcome All outcomes	High risk	By interview
Summary assessment	High risk	High