Effect of influenza vaccine on chronic warfarin therapy.

Whether influenza vaccination causes a prolongation in the prothrombin time (PT) and produces bleeding complications in patients receiving chronic warfarin therapy is a clinically important controversy. The degree of anticoagulation during chronic warfarin therapy of 33 patients was assessed before and after vaccination with trivalent types A and B influenza vaccine. Three patients were excluded because of inadequate PT data before vaccination and one patient was unavailable for follow-up after vaccination. Data from five additional patients were excluded from statistical analysis because the warfarin dose was changed within one week of vaccination. PT values obtained at zero to two weeks, three to four weeks, and two, three, and four months after vaccination were compared with each patient's prevaccination baseline value. PT values following vaccination were unchanged except for a decrease that occurred during the first two weeks (p less than 0.05). Complications did not require dosage adjustments and were limited to minor nose bleeds or bruises occurring in two patients before and three patients after vaccination. These data suggest a decrease rather than an increase in PT values following vaccination, and do not support the existence of a serious warfarin-vaccine interaction. The possibility that an occasional patient may experience such an interaction cannot be excluded, but none was seen in these patients.