Increased risk of anaphylaxis following administration of 2009 AS03-adjuvanted monovalent pandemic A/H1N1 (H1N1pdm09) vaccine.

•Quebec passive surveillance showed an increase in anaphylaxis after H1N1pdm09 vaccination.•Many cases which were reported as other AEFI also met the criteria for anaphylaxis.•Anaphylaxis was likely underrecognized and its incidence higher than passively observed.•Incidence of anaphylaxis is estimated at 13 per million doses administered.•Enhanced public awareness is unlikely to be the predominant explanation for this increase.