In the July 24, 1995, issue of the Archives, Raj et al1 reported the results of a study on the effect of influenza vaccine on the prothrombin time (PT) among patients taking oral anticoagulants (warfarin sodium). They administered the vaccine intramuscularly to 41 adult patients during the fall of 1992 and 1993 and measured the PT at baseline and at days 3, 7, and 14 after vaccination. They did not find any significant effect on the PT. Additionally, there were no clinically detectable local complications after intramuscular injection and no major or minor bleeding episodes.
However, we wish to point out that 2 of their statements need to be clarified. The authors declare, "The effect and clinical importance of influenza vaccine on PT in patients who are receiving warfarin is not clear from a review of existing literature." And also, "The present study, which to our knowledge is the
Souto JC, Oliver A, Montserrat I, Mateo J, Sureda A, Fontcuberta J. Influenza Immunization Is a Safe Procedure in Patients Undergoing Long-term Anticoagulation. Arch Intern Med. 1996;156(14):1589-1592. doi:10.1001/archinte.1996.00440130139020