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

During the last week of April 2009, a laboratory‐confirmed outbreak of pandemic A/H1N1(pH1N1) influenza was reported in an elementary school in a rural community of northern British Columbia, Canada. This school included students of Aboriginal and non‐Aboriginal background drawn from the local town (population, 2000 persons) and surrounding 5 reserves (population, 1000 persons). Laboratory confirmation of the elementary school outbreak was made on 3 May 2009, and the school was closed the following week. Because pH1N1 was first recognized in mid‐April as a novel virus, and to learn more about its characteristics, risk factors, and impact, an outbreak investigation was undertaken by public health through a school‐based telephone survey conducted between 15 May and 5 June 2009. We report findings from this investigation, which revealed a first unexpected link between prior receipt of seasonal trivalent inactivated influenza vaccine (TIV) and pH1N1‐related illness in Canada.
Investigation included 3 components: a telephone survey, laboratory surveillance for respiratory viruses, and a sero‐survey of affected households.

Telephone survey. After identification of the elementary school outbreak, the sampling frame for investigation was broadly inclusive of all households of students enrolled in the 6 local community schools. Approximately one‐half of all households in the community included at least 1 child.

Before initiating the telephone survey, letters were distributed via students to households to explain the purpose of the investigation. Trained interviewers then contacted households, obtained consent, and conducted telephone interviews with 1 adult per household, who provided information for all household members. The telephone survey was conducted between 15 May and 5 June 2009. Household information included the number of household members and sleeping rooms, Aboriginal versus non‐Aboriginal status, and whether residency was on or off a reserve. Individual‐level information included age, whether flulike symptoms were experienced and the related dates of onset, duration of time “in bed” (in days), and data on health care visits, comorbidity (corresponding to high‐risk categories specified by the National Advisory Committee on Immunization), and receipt of 2008–2009 and/or the 2007–2008 TIV.

Laboratory surveillance for respiratory viruses. All respiratory virus testing for this community was provided by the BC Centre for Disease Control (Vancouver, BC) Public Health Microbiology and Reference Laboratory (Appendix, which does not appear in the print version of the journal). During initial outbreak investigation, nasal or nasopharyngeal specimens were collected on 29 April by public health staff from a sample of students attending the affected elementary school. Thereafter, specimens were collected during the outbreak period at the clinician’s or public health official’s discretion and were evaluated as part of routine surveillance.

Sero‐survey of affected households.To validate the clinical case definition, community households with at least 1 member reporting ILI were subsequently invited to provide serum samples from both symptomatic and asymptomatic household members. An on‐site clinic was arranged during 6–8 August 2009. Antibody response to pH1N1 was quantified using the hemagglutination inhibition (HI) and microneutralization (MN) assays (Appendix). An HI threshold of 40 was used to designate seropositive versus seronegative participants, and status was confirmed using the MN assay.