Seasonal Influenza Vaccine and Increased Risk of Pandemic A/H1N1‐Related Illness. Part 3

Of the 45 participants from the affected elementary school households, 14 (31%) were seropositive for pH1N1 (13 of 14 persons with ILI). Of the 29 on‐reserve participants, 13 (45%) were seropositive for pH1N1 (12 of 13 persons with ILI). Of the 48 serum samples from participants belonging to off‐reserve households without children in the affected elementary school, 12 (25%) of 48 were seropositive for pH1N1 (8 of 12 persons with ILI).

Clinical case definitions: characteristics in relation to serologic status.To guide analyses, we assessed the sensitivity, specificity, and positive and negative predictive values of the ILI case definition in relation to HI or MN seropositive status. Parameters were highest in combination among participants belonging to households associated with the initial elementary school outbreak or who lived on a reserve. The ILI case definition had less value in predicting pH1N1 seropositive status among telephone survey participants who belonged to households other than those associated with the elementary school or who lived on a reserve (positive predictive value, 38%, 68%, and 75%, respectively)

Overall, 266 households and 1154 individuals contributed to the community telephone survey. Because respiratory virus surveillance and the follow‐up serologic survey both indicated greatest pH1N1 circulation among the originally affected elementary school and on‐reserve households, we restricted analyses of TIV effect on ILI to those households, hereafter referred to as “elementary school” or “on‐reserve” participants.

Elementary school household participants.There were 118 households with at least 1 child enrolled at the affected elementary school. Sixty‐three of these households participated in the survey, contributing data on 271 individuals. After excluding those with unknown 2008–2009 TIV status (n=17) and unknown chronic conditions (n=1), the analysis included 253 participants associated with the elementary school. Among these participants, 36 (15%) lived on one of the local reserves. Overall, 153 (60%) of 253 persons were aged
Of the 253 elementary school household participants, 66 (26%) reported ILI, with the rate highest among young children. The secondary attack rate among elementary school households was 27% (32 of 119); after excluding on‐reserve households, the rate was 20% (19 of 94). Only 1 participant reported travel to Mexico since mid‐March 2009. The proportion in the fourth quartile of household density was greater among participants with ILI than among those without ILI (40% vs 20%). The proportion of participants with chronic conditions was comparable to the BC proportion overall for children and young adults (10%). Self‐reported influenza immunization rates among non‐ILI control subjects were also comparable to BC rates estimated through other surveys among children and young adults (15%–20%) in British Colombia.