Influenza Vaccine Effectiveness in the Prevention of Influenza-Related Hospitalization in Canadian Adults Over the 2011/12 Through 2013/14 Season: A Pooled Analysis From the Serious Outcomes Surveillance (SOS) Network of the Canadian Influenza Research Network (CIRN)

Background. Ongoing assessment of influenza vaccine effectiveness (VE) is critical to inform public health decision making. The CIRN Serious Outcomes Surveillance (SOS) Network provides annual estimates of influenza VE in the prevention of influenza-associated hospitalization in adults. Here we provided pooled VE estimates across 3 influenza seasons. Methods. From 2011/12 to 2013/14, the CIRN SOS Network conducted active surveillance for influenza among hospitalized adults from ∼1 November to 30 April each season in up to 45 hospitals in 7 provinces. A nasopharyngeal swab for influenza polymerase chain reaction (PCR) was obtained from all patients admitted with any acute respiratory diagnosis or symptom. Cases were PCR-positive for influenza; test-negative controls matched for date and site of enrolment and age of the case (≥65 years versus <65 years) were enrolled for calculation of VE. VE was estimated as (1-odds ratio of influenza in vaccinated versus unvaccinated patients) × 100 for cases and controls enrolled over 3 seasons. VE estimates were adjusted using multivariable logistic regression with stepwise backward selection of covariates with a p value of <.1 in univariate analysis. Results. A total of 3394 cases and 4560 controls were enrolled; 2078 (61.2%) cases and 2939 (64.5%) controls were ≥65 years. Over 3 seasons, including all age groups, matched, adjusted VE was 41.7% (34.3–48.3%); VE in adults ≥65 years was 39.3% (29.4–47.8%) and in adults 16–64 years was 48.0% (37.5–56.7%). Including all age groups, VE against influenza A was 44.1% (35.1–51.9%) and against influenza B was 35.3% (20.7–47.3%). In adults ≥65 years, VE against influenza A/H3N2 and A/H1N1 was 24.2% (3.6–40.4) and 58.7% (39.4–71.9%), respectively. Corresponding estimates in 16–64 years were 44.4% (19.0%–61.8%) and 60.8% (45.1%–72%), respectively. Conclusion. While effectiveness of influenza vaccines to prevent serious outcomes varies year to year due to factors such as virulence and match between circulating and vaccine strains, here we demonstrate statistically and clinically important benefit of vaccination in adults spanning three seasons with an average overall effectiveness of 42%. The individual and public health benefit of influenza vaccines should not be understated and public messaging should address overall benefits over time while acknowledging year to year variability. Disclosures. S. A. McNeil, GSK: Grant Investigator, Research grant and Research support. Pfizer: Grant Investigator, Consulting fee, Research grant, Research support and Speaker honorarium. Merck: Consultant and Investigator, Consulting fee, Research support and Speaker honorarium; T. Hatchette, GSK: Investigator, Research grant; M. K. Andrew, GSK: Investigator, Research support; G. Boivin, Biocryst: Investigator, Research grant Merck: Investigator, Research grant; W. Bowie, GSK: Investigator, Research grant; A. Chit, Sanofi Pasteur: Employee, Salary; G. Dos Santos, Business and Decision Life Sciences: Consultant, Salary; F. Haguinet, GSK Vaccines: Employee, Salary; S. A. Halperin, GSK: Consultant, Grant Investigator and Research Contractor, Consulting fee and Grant recipient; B. Ibarguchi, GSK: Employee, Salary; J. M. Langley, GSK: Investigator, Research grant Sanofi Pasteur: Investigator, Research grant. PREVENT: Investigator, Research grant; P. Lagace-Wiens, Merck: Scientific Advisor, Consulting fee and Speaker honorarium; M. Loeb, GSK: Investigator, Research support; A. E. McCarthy, GSK: Investigator, Research support; J. E. McElhaney, GSK: Scientific Advisor, Research support and Speaker honorarium. Sanofi Pasteur: Scientific Advisor, Speaker honorarium; A. McGeer, GSK: Grant Investigator, Investigator and Scientific Advisor, Research support and Speaker honorarium. Sanofi Pasteur: Grant Investigator, Investigator and Scientific Advisor, Research support and Speaker honorarium. Merck: Grant Investigator, Investigator and Scientific Advisor, Research support and Speaker honorarium; A. Poirier, Actelion: Investigator, Research grant. Genetech: Investigator, Research grant. Sanofi Pasteur: Investigator, Research grant. Vertex: Investigator, Research grant; J. Powis, GSK: Investigator, Research support; V. Shinde, GSK: Employee, Salary

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