In this study, the serum PCT level of the bacteria‐positive group was significantly higher than that of the bacteria‐negative group, and the serum PCT level of the COVID‐19 patients without bacterial coinfection only increased slightly, indicating that detection of the PCT level of COVID‐19 patients can assist in diagnosing whether there is secondary bacterial infection after viral infection and may also quickly identify the severity of their infection to help determine whether antibiotics are needed to prevent sepsis. The gene discussed is CALCA; the disease is viral infectious disease.