The median ages ranged from 49 to 70.5 years old in the critical/mortal group across the enrolled studies. The median ages ranged from 37 to 62 years old in the non-critical group. The median ages were generally higher in critical/death group compared to the non-critical group. Furthermore, age over 65 years was analyzed as a subgroup by Guan and Tian (with a total amount of 1273 patients). Meta-analysis showed that the proportion of patients older than 65 years was higher in critical/death group compared to the non-critical group. There was no heterogeneity in the estimates of male, age over 65 years old and current smoking among the identified studies with I^2=0.