Phase IV, simple blind, 2-parallel groups, multicentric randomized trial where each patient will receive a single dose of the recommended by the WHO anti-flu vaccine for the 2003-2004 period, the control group will receive it subcutaneous and intervention group intramuscular. The assignation to each group will be randomized. The study included 4 basic areas of Barcelona city. Three visits have been scheduled for each patient: moment 0, 24 hours and 10 days after the vaccination, and a 7 month follow up period to control the incidence of the flu.