Herein, we report the interim vaccine effectiveness (VE) of a quadrivalent inactivated influenza vaccine, during the 2019/2020 influenza season, in Japan. We conducted a retrospective observational cohort study of 381 patients aged ≥15 years, who were enrolled with influenza like illnesses and examined via the rapid influenza diagnostic test, at the Ambulatory Care unit of the National Center for Global Health and Medicine in Tokyo, Japan, from the beginning of October 2019 to the end of January 2020. VE was estimated using a test-negative design. VE was calculated as (1 - odds ratio) × 100%, comparing influenza A test positivity between vaccinated and unvaccinated patients. Of the 381 patients initially screened for inclusion, 314 were enrolled in the study. Of these, 105 were vaccinated, 98 were diagnosed with influenza A, and 5 were diagnosed with influenza B. Overall VE against influenza A was 27.6% (95% confidence interval [CI], ‒21.1 to +57.4), and in patients aged ≥65 years, it was 47.3% (95% CI, ‒76.0 to +86.0). This indicates that the influenza vaccination offered continued protection during the 2019/2020 influenza season, but a detailed analysis of more cases with a careful consideration of methodology is necessary to estimate VE more precisely.
Keywords: influenza; test-negative design; vaccine effectiveness.