Risk of seasonal influenza by occupation in a railway company in a metropolitan area of Japan during three influenza seasons

Station staff may be at high risk for influenza due to high frequency of contacts with other persons. We examined the risk of influenza by occupational group in a railway company. A retrospective observational study was conducted among employees at a branch office of a railway company in eastern Japan, located in a metropolitan area, for each of the 2012/13, 2013/14, and 2014/15 influenza seasons. The study population was employees who had received an influenza vaccination for the season in question and the previous season. Outcome was defined as self-reported influenza illness during the relevant season, identified through the vaccine screening questionnaire in the next season. Study participants were employees from whom we were able to obtain outcome information. Standardized morbidity ratios (SMRs) by occupational group (station staff, engineers, train crew) for each season were calculated. For the 2012/13, 2013/14, and 2014/15 seasons, attack rates were 4.7% (19/403), 5.2% (21/407), and 7.8% (31/397), respectively. Compared with all participants, SMRs of station staff were lower in the 2012/13 (SMR=57; 95%CI=18-133) and 2014/15 (SMR=75; 95%CI=36-138) seasons and similar in the 2013/14 season. Enhanced countermeasures, regardless of occupational group, may be effective in preventing the spread of influenza infection.