Background: We estimate vaccine effectiveness (VE) against both influenza A/subtypes and B/lineages in Canada for the 2011-2012 trivalent inactivated influenza vaccine (TIV) with components entirely unchanged from the 2010-2011 TIV and in the context of phenotypic and genotypic characterization of circulating viruses.
Methods: In a test-negative case-control study VE was estimated as [1-(adjusted)OddsRatio] × 100 for RT-PCR-confirmed influenza in vaccinated vs nonvaccinated participants. Viruses were characterized by hemagglutination inhibition (HI) and sequencing of antigenic sites of the hemagglutinin (HA) gene.
Results: There were 1507 participants. VE against A(H1N1)pdm09 was 80% (95% confidence interval [CI], 52%-92%): circulating viruses were HI-characterized as vaccine-matched and bore just 2 aminoacid (AA) differences from vaccine. VE against A/H3N2 was 51% (95% CI, 10%-73%): circulating viruses were HI-characterized as vaccine-related but bore ≥11AA differences from vaccine. VE against influenza B was 51% (95% CI, 26%-67%) in total: 71% (95% CI, 40%-86%) for lineage-matched B/Victoria and 27% (95% CI, -21% to 56%) for lineage-mismatched B/Yamagata. For both influenza A and B types, VE was similar among recipients of either 2010-2011 or 2011-2012 TIV alone, higher when vaccinated both seasons.
Conclusions: Phenotypic and genotypic characterization of circulating and vaccine viruses enhances understanding of TIV performance, shown in 2011-2012 to be substantial against well-conserved A(H1N1)pdm09 and lineage-matched influenza B, suboptimal against genetic-variants of A/H3N2, and further reduced against lineage-mismatched influenza B. With unchanged vaccine components, protection may extend beyond a single season.