Seasonal Influenza Vaccine Effectiveness in Preventing Laboratory Confirmed Influenza in 2014-2015 Season in Turkey: A Test-Negative Case Control Study

BACKGROUND: Influenza has an important public health impact worldwide with its considerable annual morbidity among persons with or without risk factors and its serious complications among persons in high-risk groups. The seasonal influenza vaccine is essential for preventing the burden of influenza in a population. Since the vaccine is reformulated each season according to the virus serotypes in circulation, its effectiveness can vary from season to season. Vaccine effectiveness is defined as the relative risk reduction in vaccinated individuals in observational studies. AIMS: To calculate influenza vaccine effectiveness in preventing laboratory-confirmed influenza in the Turkish population for the first time using the national sentinel surveillance data in the 2014-2015 influenza season. STUDY DESIGN: Test-negative case-control study. METHODS: We compared vaccination odds of influenza positive cases to influenza negative controls in the national influenza surveillance in Turkey to estimate influenza vaccine effectiveness. RESULTS: The influenza vaccine effectiveness against influenza A (H1N1) (68.4%, 95% CI: -2.9 to 90.3) and B (44.6%, 95% CI: -27.9 to 66.6) were moderate, and the influenza vaccine effectiveness against influenza A (H3N2) (75.0%, 95% CI: -86.1 to 96.7) was relatively high; all had low precision given the low vaccination coverage. Overall, the influenza vaccination coverage rate was 4.2% (95% CI: 3.5 to 5.0), which is not sufficient to control the burden of influenza. CONCLUSION: In Turkey, national surveillance for influenza should be strengthened and utilised annually for the assessment of influenza vaccine effectiveness with more precision. Annual influenza vaccine effectiveness in Turkey should continue to be monitored as part of the national sentinel influenza surveillance.