Warfarin Institute of America
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INFLUENZA VIRUS VACCINE INTERACTIONS WITH WARFARIN
There is conflicting information as to whether or not this interaction occurs. A confounding factor may be that the composition if the flu vaccine is different every year. There has also been a great improvement in purifying the vaccines since the studies were done. Perhaps some formulations cause it to occur and others do not. The three articles quoted here are all more than fifteen years old. Also, there have been improvements in monitoring warfarin therapy, most notably the use of the INR system, since these studies were done.
Kramer et al. reported on two conflicting studies -- one show an effect in actual patients and one showing no effect in healthy test subjects. Weibert et al. reported on two studies in actual patients. One showed a rise in the anticoagulation level and one did not. Gomolin compared seven elderly patients on warfarin who received influenza vaccine with nine elderly patients who were not taking warfarin and received the vaccine. The was no difference in change in the prothrombin time between the groups.
References:
Kramer P et al. Effect of influenza vaccine on warfarin anticoagulation. Clin Pharmacol Ther 1984;35:416-418.
Weibert RT et al. Effect of influenza vaccine in patients receiving long-term warfarin therapy. Clin Pharm 1986;5:499-503.
Gomolin IH. Lack of effect of influenza vaccine on warfarin anticoagulation in the elderly. CMAJ 1986;135:39-41.
My experience and opinion:
With the number of elderly people taking warfarin and getting flu shots every year, this would be much more reported if something significant happened. I do not routinely monitor people after the have had flu shots. Occasionally I will see someone who has a elevated INR that we cannot explain. If it is in the "flu shot season" quite often it turns out that they had a shot in the past 48 hours. I usually have them hold one warfarin doses and return to their regular warfarin dosage. I do not know of anyone who experienced any ill effect doing this.
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Last updated November 13, 2004