CEACAM3 and COVID-19: Furthermore, in order to identify the association between CEA levels and smoking, two subgroup Cox proportional hazard regression models including smoking status (keep or remove missing values) were constructed, suggesting that the CEA level (HR 0.547; 95% CI 0.318 to 0.940; P = 0.037) (remove missing values) (HR 0.620; 95% CI 0.384 to 0.990; P = 0.048) (keep missing values) was still an independent prognostic indicator for COVID-19 patients (SAdditiona file 4: Supplementary material 3).