Lack of effect of influenza vaccine on warfarin anticoagulation in the elderly.

Influenza vaccine may inhibit hepatic metabolism of drugs and has been reported to prolong the prothrombin time in patients receiving warfarin by affecting the coagulation pathway. In a group of seven elderly residents of a long-term care facility who were receiving warfarin, prolongation of the prothrombin and partial thromboplastin times could not be shown up to 5 weeks after vaccination against influenza. The results were similar in nine elderly subjects who were not receiving warfarin.