In this retrospective study of 822 COVID-19 cases, three major observations were demonstrated: (i) cardiac injury was prevalent in diabetic patients with COVID-19, and conferred a nearly 13-fold higher risk of in-hospital mortality in those people; (ii) COVID-19 patients with DM and cardiac injury present much severer immune-inflammatory responses; and (iii) decreased number of CD3+CD4+ T cells and increased IL-6 value particularly refer high risk of cardiac injury in diabetic COVID-19 patients. The gene discussed is IL6; the disease is diabetes mellitus.