Interim estimates of 2014/15 influenza vaccine effectiveness in preventing laboratory-confirmed influenza-related hospitalisation from the serious outcomes surveillance network of the Canadian immunization research network, January 2015

The 2014/15 influenza season in Canada has been characterised to date by early and intense activity dominated by influenza A(H3N2). A total of 99.0% (593/599) hospitalisations for laboratory-confirmed influenza with a known influenza virus type enrolled in sentinel hospitals of the Serious Outcomes Surveillance Network of the Canadian Immunization Research Network were due to influenza A. Of the 216 with a known subtype, influenza A(H3N2) accounted for 99.1% (n=214). Interim unmatched vaccine effectiveness (VE) estimates adjusted for age and presence of one or more medical comorbidities were determined by test-negative case-control design to be -16.8% (90% confidence interval (CI): -48.9 to 8.3) overall and -22.0% (90% CI: -66.5 to 10.7) for laboratory-confirmed influenza A(H3N2). Among adults aged under 65 years, the overall VE was 10.8% (90% CI: -50.2 to 47.0) while in adults aged 65 years or older, the overall VE was -25.4% (90% CI: -65.0 to 4.6).