The worldwide spread of coronavirus disease 2019 (COVID-19) has resulted in an unparalleled health emergency of global proportions. Around 31% of individuals with COVID-19 experience thrombosis associated with hypercoagulation. COVID-19 patients have shown an increase in platelet activation, but the mechanism has not been fully understood yet. One theory suggests that this could be related to the heparin-induced thrombocytopenia phenomenon, where platelet activation involves anti-PF4 antibodies that are associated with thrombosis. Vitamin D has been established to exert an influence on immunological responses and inflammation. The aim of this study is to analyze the correlation between serum 25-hydroxy-cholecalciferol [25 (OH) D] levels and anti-PF4 antibodies among COVID-19 patients. A cross-sectional study was conducted among 160 COVID-19 patients at Cipto Mangunkusumo General …