Estimates of Influenza Vaccine Effectiveness for 2007–2008 From Canada’s Sentinel Surveillance System: Cross-Protection Against Major and

Objectives. To estimate influenza vaccine effectiveness (VE) for the 2007–2008 season and assess the sentinel surveillance system in Canada for monitoring virus evolution and impact on VE. Methods. Nasal/nasopharyngeal swabs and epidemiologic details were collected from patients presenting to a sentinel physician within 7 days of influenza-like illness onset. Cases tested positive for influenza A/B virus by real-time polymerase chain reaction; controls tested negative. Hemagglutination inhibition (HI) and gene sequen-cing explored virus relatedness to vaccine. VE was calculated as 1 minus the odds ratio for influenza in vaccinated versus nonvaccinated participants, with adjustment for confounders. Results. Of 1425 participants, 21 % were vaccinated. Influenza virus was detected in 689 (48%), of which iso-lates from 663 were typed/subtyped: 189 (29%) were A/H1, 210 (32%) were A/H3, and 264 (40%) were B. Of A/H1N1 isolates, 6 % showed minor HI antigenic mismatch to vaccine, with greater variation based on genetic identity. All A/H3N2 isolates showed moderate antigenic mismatch, and 98 % of influenza B virus isolates showed major lineage-level mismatch to vaccine. Adjusted VE for A/H1N1, A/H3N2, and B components was 69 % (95% confidence interval [CI], 44%–83%), 57 % (95 % CI, 32%–73%), and 55 % (95 % CI, 32%–70%), respectively, with an overall VE of 60 % (95 % CI, 45%–71%). Conclusions. Detailed antigenic and genotypic analysis of influenza viruses was consistent with epidemiologi