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Background. In April 2009, an elementary school outbreak of pandemic H1N1 (pH1N1) influenza was reported in a community in northern British Columbia, Canada—an area that includes both non-Aboriginal and Aboriginal residents living on or off a reserve. During the outbreak investigation, we explored the relationship between prior receipt of trivalent inactivated influenza vaccine (TIV) and pH1N1-related illness. Methods. A telephone survey was conducted from 15 May through 5 June 2009 among households of children attending any school in the affected community. Members of participating households where influenza-like illness (ILI) was described were then invited to submit blood samples for confirmation of pH1N1 infection by hemagglutination inhibition and microneutralization assays. Circulation of pH1N1 was concentrated among households of the elementary school and elsewhere on-reserve to which analyses of TIV effect were thus restricted. Odds ratios (ORs) for the TIV effect on ILI were computed through logistic regression, with adjustment for age, comorbidity, household density, and Aboriginal status. The influence of within-household clustering was assessed through generalized-linear-mixed models. Results. Of 408 participants, 92 (23%) met ILI criteria: 29 (32%) of 92 persons with ILI, compared with 61 (19%) 316 persons without ILI, had received the 2008-2009 formulation of TIV. Fully adjusted ORs for 2008- 2009 TIV effect on ILI were 2.45 (95% confidence interval, 1.34-4.48) by logistic regression and 2.68 [95% confidence interval, 1.37-5.25) by generalized-linear-mixed model. Conclusions. An outbreak investigation in British Columbia during the late spring of 2009 provided the first indication of an unexpected association between receipt of TIV and pH1N1 illness. This led to 5 additional studies through the summer 2009 in Canada, each of which corroborated these initial findings. [ABSTRACT FROM AUTHOR]