Influenza vaccine effectiveness against influenza-related hospitalization during a season with mixed outbreaks of four influenza viruses: a test-negative case-control study in adults in Canada
Creators: name: Andrew, Melissa K.
name: Shinde, Vivek
name: Hatchette, Todd
name: Ambrose, Ardith
name: Boivin, Guy
name: Bowie, William
name: Chit, Ayman
name: Dos Santos, Gael
name: ElSherif, May
name: Green, Karen
name: Haguinet, François
name: Halperin, Scott A.
name: Ibarguchi, Barbara
name: Johnstone, Jennie
name: Katz, Kevin
name: Langley, Joanne M.
name: LeBlanc, Jason
name: Loeb, Mark
name: MacKinnon-Cameron, Donna
name: McCarthy, Anne
name: McElhaney, Janet
name: McGeer, Allison
name: Nichols, Michaela K.
name: Powis, Jeff
name: Richardson, David
name: Semret, Makeda
name: Stiver, Grant
name: Trottier, Sylvie
name: Valiquette, Louis
name: Webster, Duncan
name: Ye, Lingyun
name: McNeil, Shelly
Publisher: name: BioMed Central
Year: 2018

description: Background The Serious Outcomes Surveillance (SOS) Network was established to monitor seasonal influenza complications among hospitalized Canadian adults and to assess the effectiveness of influenza vaccination against severe outcomes. Here we report age- and strain-specific vaccine effectiveness (VE) in preventing severe outcomes during a season characterized by mixed outbreaks of four different influenza strains. Methods This prospective, multicentre, test-negative case-control study evaluated the VE of trivalent influenza vaccine (TIV) in the prevention of laboratory-confirmed influenza-hospitalization in adults aged ≥16 years (all adults) and adults aged 16–64 years (younger adults). The SOS Network identified hospitalized patients with diagnoses potentially attributable to influenza during the 2011/12 influenza season. Swabs collected at admission were tested by reverse transcriptase polymerase chain reaction (RT PCR) or viral culture to discriminate influenza cases (positive) from controls (negative). VE was calculated as 1-odds ratio (OR) of vaccination in cases versus controls × 100. Results Overall, in all adults, the unadjusted and adjusted VEs of TIV against influenza-hospitalization were 41.8% (95% Confidence Interval [CI]: 26.0, 54.3), and 42.8% (95% CI: 23.8, 57.0), respectively. In younger adults (16–64 years), the unadjusted and adjusted VEs of TIV against influenza-hospitalization were 35.8% (95% CI: 4.5, 56.8) and 33.2% (95% CI: −6.7, 58.2), respectively. In the all adults group, adjusted VE against influenza A/H1N1 was 72.5% (95% CI: 30.5, 89.1), against A/H3N2 was 86.1% (95% CI: 40.1, 96.8), against B/Victoria was 40.5% (95% CI: −28.9, 72.6), and against B/Yamagata was 32.3% (95% CI: −8.3, 57.7). The adjusted estimate of early season VE (from November 1 to March 11) was 54.4% (95% CI: 29.7–70.4), which was higher than late season (from March 11 to May 25) VE estimate (VE: 29.7%, 95% CI: -5.3, 53.1). Conclusions These results suggest that TIV was highly effective against A viruses and moderately effective against B viruses during a mild season characterised by co-circulation of four influenza strains in Canada. Findings underscore the need to provide VE assessment by subtype/lineage as well as the timing of vaccination (early season vs late season) to accurately evaluate vaccine performance and thus guide public health decision-making. Trial registration ClinicalTrials.gov Identifier: NCT01517191. Registration was retrospective and the date of registration was January 17, 2012.