Effect of influenza vaccination on international normalized ratio during chronic warfarin therapy.

ABSTRACT

WHAT IS KNOWN AND OBJECTIVE: Warfarin is a widely used anticoagulant, well-known for its interactions with medications and foods. Vaccinations, particularly the influenza vaccine, have been thought to potentially interfere with anticoagulation response in those on chronic warfarin. Our objective was to systematically review the literature to assess the validity and clinical significance of this association. METHODS: A primary literature search was performed using MEDLINE (1966 - June 2011) and EMBASE (1980 - June 2011). Additional studies were obtained by performing a manual bibliographical review of literature from the initial results and by searching The Cochrane Database of Systematic Reviews. All English-language, peer-reviewed publications identified were evaluated. Reviews, case studies and trials reporting anticoagulation response using an unconverted prothrombin time ratio were excluded. RESULTS AND DISCUSSION: Thirty-one abstracts were initially reviewed, and seven studies were identified for inclusion in this review. Significant changes in mean INR post-vaccination between the study and comparator groups were documented in one trial. Through subgroup analysis, another study noted that elderly patients spent more time in the subtherapeutic range post-vaccination when compared with baseline INR levels. No other significant changes in mean INR levels were documented following influenza vaccination. Adverse bleeding events reported after immunization were limited and minor in nature. WHAT IS NEW AND CONCLUSION: Overall, our review does not indicate a consistent, clinically relevant effect of influenza vaccines on INR of patients on chronic warfarin therapy. Isolated reports of variations in INR following influenza vaccination are likely due to other factors.