Until now, the best administration route of the anti-flu vaccine for patients receiving long term oral anticoagulation therapy has been the subcutaneous one; the intramuscular route has not been recommended because it may increase the risk of muscular haematoma. Although this practice is widely extended, no significant differences between the safety of both routes have been found. Some authors, analysing a small group of patients, affirm that the anti-flu vaccine can be administered safely by the intramuscular route on patients receiving long term oral anticoagulation therapy, while a previous opinion article recommended the subcutaneous route without any scientific evidence. Due to this, the investigators have done a clinical trial with the aim of determining the safety and effectiveness of the flu-vaccine intramuscular route in patients receiving long term oral anticoagulation therapy and also to analyse the possible interactions between the vaccine and the oral anticoagulant treatment.

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.

Inclusion Criteria:

• Patients with oral anticoagulation therapy
• Patients who agree to be vaccinated
• Patients who agree to participate in the study
• Patients older than 18

Exclusion Criteria:

• Allergy or hypersensibility to egg, chicken proteins, any component of the vaccine (neomycin, octoxinol-9 or formaldehyd)
• INR greater than 4 in the past 2 months
• Previous major bleeding
• Patients with dementia or with legal tutoring