Effectiveness of seasonal influenza vaccine in adult Japanese workers, 2017-2020

Abstract

Background: Previous studies have not estimated vaccine effectiveness (VE) against influenza in the working-age Japanese population. In this study, we determined VE in adult workers at a Japanese company.

Methods: We estimated VE based on self-reported data regarding influenza infections and vaccinations in employees of an auto parts manufacturing company during three influenza seasons from 2017 to 2020. VE was estimated as 100% × [1 - odds ratio (the ratio of the odds of being diagnosed with influenza among enrollees with and without influenza vaccination)]. Odds ratios were estimated using logistic regression.

Results: We included 11,347 worker records [3,592 (2017-18), 3,663 (2018-19), and 4,092 (2019-20)] from employees who had worked with the company throughout each influenza season. The adjusted VE was moderate and significant in the 2019-20 season (VE = 53%; 95% confidence interval [CI] = 30% to 69%) but low or negative and non-significant during the 2017-18 (VE = 28%; 95% CI = -5% to 50%) and 2018-19 (VE = -11%; 95% CI = - 42% to 14%) seasons.

Conclusions: Influenza vaccines were moderately effective during the 2019-20 season but showed low or negative effectiveness during the 2017-18 and 2018-19 seasons. Self-reports from worker records can successfully help determine VE against influenza.

Keywords: Adults; Influenza; Influenza vaccines; Vaccine effectiveness; Workers.