Lack of influence of SARS-CoV 2 vaccination on anticoagulation levels of patients on stable, long-term warfarin treatment

Dear Sir,

We appreciate the interest of Rujittika Mungmunpuntipantip and Viroj Wiwanitkit¹ in our study. We agree that there are no data suggesting biological mechanisms of COVID-19 infection that could be linked to the pharmacological action of warfarin. However, when we designed the study we aimed to test the hypothesis that general variations in health status of patients due to an inflammatory reaction to vaccination could modify the response to warfarin treatment. It is known that the determinants of the quality of anticoagulant therapy are influenced by associated diseases, such as heart failure, diabetes and intercurrent illness^2–4.

Mild to moderate illness has been reported after COVID-19 vaccination, with the most commonly reported systemic events being fatigue and headache, occurring in more than 50% of patients. Moreover, fever and chills were observed in a minority of patients after vaccination⁵. Our aim was, therefore, to evaluate whether the commonly mild reactions to vaccination could alter the response to warfarin. Our data cannot give any information about biological mechanisms related to immunological responses, as suggested by Mungmunpuntipantip and Wiwanitkit. Specifically, we did not perform laboratory tests aimed at identifying possible conditions interfering with determination of the International Normalized Ratio, and our study should be considered of purely clinical relevance.