Effect of influenza vaccine in patients receiving long-term warfarin therapy

The effect of influenza vaccine on anticoagulant response in 12 patients receiving long-term warfarin therapy was studied. Study criteria required that all patients have stable prothrombin times and be on stable warfarin dosages before immunization. Patients were immunized with 1982-83 trivalent influenza vaccine (subvirion), types A and B 0.5 mL i.m. on day 0 of the study. Prothrombin times were determined on days--7, 0, 2, 5, 7, 14, and 21, and results were reported as ratios with control values. Influenza immunization produced a small but significant increase in the prothrombin-time ratio. Before immunization the mean ratio was 1.68, and the mean ratio increased to 1.81 after immunization. The maximal increase occurred on day 14 and represented a 7.6% increase over the baseline (day 0) value. The prothrombin-time ratio was not influenced by warfarin sodium dosage (less than or equal to 5 mg/day versus greater than 5 mg/day) or by the sex of the patient. During the 1984-85 influenza season, an additional 26 patients were immunized with the 1984-85 trivalent influenza vaccine (subvirion), types A and B. The prothrombin-time ratio on days 0 and 14 were not significantly different in these patients. Although the administration of influenza vaccine to patients receiving long-term treatment with warfarin appears to be safe, patients should be monitored after immunization for possible increases in anticoagulant response.