In additional sensitivity analyses based on logistic regression and restricted to the period of peak ILI activity (27 April–11 May 2009), the overall OR for 2008–2009 TIV effect adjusted for age, chronic conditions, Aboriginal status, and household density was higher (3.55; 95% CI, 1.70–7.34). When we used control subjects defined as fully asymptomatic persons rather than those who merely lacked ILI, the overall adjusted OR was 2.51 (95% CI, 1.3–4.82), comparable to the primary analysis.
In further sensitivity analyses based on logistic regression and restricted to participants without chronic conditions, the OR for 2008–2009 TIV effect adjusted for age, Aboriginal status, and household density was 3.44 (95% CI, 1.80–6.59). In an analysis further restricted to on‐reserve participants without chronic conditions, the OR for the 2008–2009 TIV effect adjusted for age and household density was 5.38 (95% CI, 2.27–12.75). In analysis restricted to non‐Aboriginal households of the elementary school population, the OR adjusted for age, chronic conditions, and household density was 1.83 (95% CI, 0.56–5.93).
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ORs for 2007–2008 TIV effect were similar or slightly higher at 3.08 (95% CI, 1.71–5.58) overall. Analyses comparing the effect of having received 2007–2008 TIV only, 2008–2009 TIV only, or both the 2007–2008 and 2008–2009 vaccines versus no TIV either year yielded ORs of 2.36 (95% CI, 0.95–5.84), 1.31 (95% CI, 0.40–4.31), and 3.39 (95% CI, 1.7–6.68), respectively. However, most vaccinated participants had received vaccine in both 2007–2008 and 2008–2009, such that per‐season results include small sample sizes and should be interpreted cautiously: 69 (69%) of 100 recipients of 2007–2008 TV were revaccinated in 2008–2009, and 69 (76%) of 89 recipients of 2008–2009 TV had been vaccinated in 2007–2008.
Sample size was small and 95% CIs intervals were wide, but the same trend in point estimates for TIV effects was observed, with a fully adjusted OR of 2.07 (95% CI, 0.31–14.03) for 2008–2009 TIV and of 2.71 (95% CI, 0.4–18.51) for 2007–2008 TIV.
In this article, we present the first observation of an unexpected association between prior seasonal influenza vaccination and pH1N1 illness during the spring and summer of 2009 in Canada. Specifically, outbreak investigation conducted during the early stages of the pandemic in a northern BC community identified that participants reporting pH1N1‐related ILI during the period 1 April through 5 June 2009 were more than twice as likely to report having previously received seasonal influenza vaccine.
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Advantages offered by the original outbreak investigation described here include active and standardized inquiry about pH1N1‐related illness, TIV receipt, and relevant covariates for all household members, obviating potential selection biases associated with differential health care access or health care–seeking behavior in other methods of participant recruitment or case detection. By restricting analysis to households with children in a single community and in groups among whom pH1N1 circulation was confirmed serologically, we ensured the population at risk was well circumscribed and that control subjects were drawn from the same source population as cases, further minimizing the risk of selection bias. To account for influential covariates, we used recognized analysis techniques of restriction as well as adjustment for age, comorbidity, household density, and Aboriginal status.