On‐reserve household participants.Two hundred twenty‐four individuals who participated in the community survey were reported as living on a surrounding reserve. After excluding individuals with unknown 2008–2009 TIV status (n=29), unknown chronic condition (n=1), or age 6 months on 31 December 2008 (n=4), the on‐reserve analysis included 191 survey participants, of whom 100 (52%) were aged <20 years and 26 (14%) were aged 50 years. Of the 191 on‐reserve participants, 44 (23%) reported having ILI, again with the rate highest among young children. The secondary attack rate among on‐reserve households was 24% (23 of 94). None of these participants reported travel to Mexico since mid‐March. The proportion of participants living in households in the fourth quartile of density was greater for on‐reserve participants (39%) than for elementary school participants (25%) and among on‐reserve participants with ILI (50%) than among those without ILI (35%). A higher proportion of on‐reserve participants with ILI sought medical care (25 [57%] of 44), compared with elementary school participants (25 [38%] of 66). A comparable proportion for whom information was available sought care 48 h after onset (3 [20%] of 15 vs 3 [19%] of 16). No participant in either population was prescribed antivirals, and none were hospitalized.
A higher proportion of Aboriginals in British Colombia and Canada, compared with the general population, have at least 1 chronic condition. Accordingly, the proportion of young on‐reserve participants in our study with at least 1 chronic condition (27 [14%] of 191) was higher than the proportion of elementary school participants and was within the expected range overall for young Aboriginals who lived on a reserve.
Influenza vaccination rates were higher among Aboriginals who lived on a reserve in British Colombia overall (45%) than among the general population (30%). Thus, among non‐ILI control subjects in our survey, young on‐reserve Aboriginals had higher influenza immunization rates (42 [29%] of 147 persons) than did elementary school household participants (26 [14%] of 187 persons).
Effect of Seasonal Influenza Vaccine on pH1N1 Risk
Fully adjusted overall and stratified ORs for the effect of 2008–2009 TIV on ILI risk are shown in Table 3. Fully‐adjusted OR overall was 2.45 (95% confidence interval [CI], 1.34–4.48) when estimated by logistic regression and 2.68 (95% CI, 1.37–5.25) by GLMM. ORs were higher with restriction to younger participants aged <20 years or <50 years and to on‐reserve participants, especially with similar age restriction (Table 3). Although ORs were higher among Aboriginals who lived on a reserve than among non‐Aboriginal participants, an analysis for interaction did not reach statistical significance (P>.05).