Reduced efficiency of influenza vaccine in prevention of influenza-like illness in working adults: a 7 month prospective survey in EDF Gaz de France employees, in Rhone-Alpes, 1996-1997

The efficiency of influenza vaccine was evaluated in the working population by comparing the percentage of people presenting with an influenza-like illness (ILI) according to their influenza immunization status, drug expenses and workdays lost. A self-completed questionnaire about the vaccination was sent to 5785 people randomly chosen among 18 249 workers. When any sick leave was incurred amongst the respondents (63.3%), of whom 301 were vaccinated and 3362 unvaccinated, a clinical form was completed by the private physician and the medical adviser of the firm (Electricité de France and Gaz de France). A final self-completed questionnaire was sent to people whose sick leave was not documented by a physician's reported diagnosis. In total, we obtained complete data for 90.9% of the sampling. The vaccine coverage rate of 8.2% [95% confidence interval (95% CI) = 7.4-9.0%] was higher in men than in women, increasing with age and professional category. Among the 775 subjects with a medical diagnosis, the vaccine effectiveness was not significant: 27.3% (95% CI = -13.8 to 53.5%). In the unvaccinated group, 9.6% had days absent from work, versus 7.0% in the vaccinated group; the two populations were comparable in terms of clinical symptoms, smoking habits, exposure to respiratory risk factors and chronic pathology. The average duration of sick leave for ILI was not significantly different between vaccinated (0.5 days) and unvaccinated workers (0.6 days). Despite the large size of the population and the occurrence of an epidemic due to a virus closely related to the vaccine strain (A/Wuhan/359/95), the vaccine did not effectively protect the small vaccine group nor result in an economic benefit, whatever the professional group.