The development of anti‐CCP antibodies depends on various genetic factors, including shared epitope (SE), the alleles of HLA‐DRB1, and polymorphism in peptidyl arginine deiminase 4 (PADI4), as well as environmental factors such as smoking and infection with Porphyromonas gingivalis, the agent of periodontitis which may also affect the development of anti‐CCP in COVID‐19 patients with different disease severity.32, 33, 34, 35. Here, PADI4 is linked to periodontitis.