Influenza vaccination and warfarin anticoagulation.

Because influenza vaccine can markedly depress hepatic cytochrome P450 activity and may have caused an extreme prolongation of the prothrombin time in one patient receiving warfarin, we studied the effect of influenza vaccine on anticoagulation in 21 male outpatients receiving chronic warfarin therapy. Prothrombin times measured three to five times in the month after vaccination were in the therapeutic range as often as those measured before vaccination. Mean changes in prothrombin times measured after vaccination were not significantly different from those on the day of vaccination or in the 3 months before vaccination. The size of this study assured an 80% chance that any deviation from the observed mean prothrombin times would not exceed 2.1 s. No patient had any bleeding or thromboembolic problems. Thus, influenza vaccination of patients anticoagulated with warfarin appears to be safe.