COVID-19 vaccination and anticoagulation: Potential effects on anticoagulation control in patients using vitamin kantagonists

Background : In January 2021, the Dutch vaccination programme against SARS-CoV-2 was started. Clinical studies have shown that systemic reactions including fever and chills occur in up to 50% of vaccine recipients. It is unclear whether these systematic reactions have an effect on anticoagulation control, potentially leading to an increased risk of thrombotic events and bleeding complications. Aims : To investigate whether COVID-19 vaccination with the Pfizer vaccine is associated with suboptimal anticoagulation control in patients using Vitamin K antagonists (VKAs). Methods : A case-crossover study was performed in a cohort of outpatients from three Dutch anticoagulation clinics who received a COVID-19 vaccination. All patients had their international normalized ratio (INR) measured 0-6 weeks before and 1-2 weeks after vaccination. INR results and VKA dosages before the first COVID-19 vaccination, the reference period, were compared with those after the first vaccination. Data extraction after the second vaccination will be performed in the near future, after which these analyses will be repeated and included in the final presentation at the congress. Results : A total of 2197 outpatient VKA-users were included, with a mean age of 86 years, of whom 38.5% were male and 71.7% used acenocoumarol (Table 1). There was no difference in mean INR level before and after vaccination (2.51 vs 2.54, mean difference 0.033 (95% CI, -0.071 to 0.0051). The mean dosage of phenprocoumon did not differ before and after vaccination (0.47 tablets/day (0.25)). Similarly, the mean dosage of acenocoumarol was comparable before and after vaccination (1.72 tablets/day (0.81) versus 1.71 tablets/ day (0.82). Most vaccine recipients remained in therapeutic range and INR > 5 was as likely to be reported after vaccination (1.0% and 2.6%) as it was before vaccination (1.0% and 1.6%) (Table 2). Conclusions : COVID-19 vaccination did not influence anticoagulation control in patients using VKAs.