This paper is a specific cost-benefit study concerning 25-64 years people flu vaccination. For this active population the vaccination rate is only 12% and the cost of absenteeism is potentially high. To make the balance between prevention costs and (direct and indirect) avoided costs by the vaccine, we use a basic case with mean value parameters as: efficacy rate of vaccine, impact of epidemics, size of vaccinated population, immunization length and compensation rate of the so-called production losses. These parameters are then supposed to vary in a sensitivity analysis. In any case net benefit of vaccination appears, which size varies mainly in relation to epidemics and vaccinated population extents. But improvements are to be achieved for giving more precise values to "real" efficacy rate of vaccination and effective economic impact of absenteeism on production.