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Project #44:
Pre-Covid: Anti-coags


(1) It was reported that people with severe COVID-19 are at risk of blood clots. Warfarin is a commonly used blood thinning treatment. A previous study suggested that it might increase the risk of severe COVID-19 related outcomes due to the process of how it produces an effect in the human body to prevent blood clots, which is different from its alternative treatment (direct oral anticoagulants, DOACs). However, little is known about the impact of routine use of warfarin on COVID-19 related blood clots.

Therefore, we studied a large number of people with atrial fibrillation (a medical condition of an irregular heart rhythm that can lead to blood clots in the heart) who were prescribed warfarin or DOACs as part of normal clinical care and followed them up to compare the effects of warfarin on several COVID-19 related outcomes with DOACs.

Overall, this study shows no evidence of harmful effects of warfarin on severe COVID-19 disease, compared with DOACs. This study provides reassurance about the safety of warfarin use among patients with atrial fibrillation in the context of the COVID-19 pandemic.

(2) Early studies showed that blood thinning treatment (anticoagulants) reduce the risk of thrombotic events amongst those infected. However, current evidence of the role of routinely prescribed oral anticoagulants (OACs) in COVID-19 outcomes is unclear.

Therefore, we studied a large number of people with atrial fibrillation (a medical condition of an irregular heart rhythm that can lead to blood clots in the heart) who had a low risk of stroke. Among this study cohort, the decision of prescribing OAC would typically be borderline. We followed them up to compare the effects of those who prescribed oral anticoagulants on several COVID-19 related outcomes with those who did not .

In conclusion, this study shows that people receiving OACs had a lower risk of testing positive for SARS-CoV-2 and severe COVID-19 outcomes than non-users. This might be explained by the potential effect of OACs in preventing severe COVID-19 outcomes or more cautious behaviours leading to reduced infection risk.


  • Study leads: Angel Wong
  • Organisation: NHS England (in collaboration with University of Oxford and LSHTM)