Bivariate analyses showed that some complications in COVID-19 patients (e.g., heart failure, septic shock, respiratory failure, acute cardiac and kidney injury, and secondary infections) were mainly associated in a non-linear manner with both pre-existing conditions (such as age, comorbidities, diabetes, and cerebrovascular disease) and blood parameters (such as PLT, CK, and IL-6). Here, IL6 is linked to respiratory failure.