[The concept of risk in influenza (author's transl)]

The evaluation of morbidity risk is difficult in influenza because of a lack of accurate quantitative information and of a high variability of the virus. The level of specific herd immunity towards epidemic strains is an important factor of prevision. As far as mortality is concerned, there is a net and regular increase of risk up from the age of 45. In addition to elderly, other high risk categories are known such as patients suffering of a number of organic deficiences or chronical conditions, pregnant women and children. The best evaluation of efficacy of vaccines is the study of incidence reduction in a vaccinated group in comparison to unvaccinated controls. Present vaccines are really efficient to reduce the risk of getting the disease, except in the case of an antigenic shift, which means the activity of a new antigenic component. These advantages are to be weighted to the small risk of vaccine reactions, which are not frequent and usually benign. Several vaccination strategies can be used. In France, it has been decided to recommand vaccine to priority groups: people over 65 and deficient patients. This policy is eqlivalent in many western countries (West Europe, USA); it is not efficient in preventing or delaying epidemics.