2 Epidemiological Characteristics

Based on the cases that have been searched for in the early COVID-19 spread, the descriptions of its epidemiology in the first edition were limited to that “most cases treated currently have a exposure history of Wuhan Huanan seafood market, and some cases appear in family aggregation”.

An “Epidemiological Characteristics” section was added and described sep arately in the fourth edition. The source of infection, route of transmission and susceptible individuals were illustrated respectively: “The major source of infection is the patients with COVID-19, and it is mainly transmitted by respiratory droplets, also via contact route. The elderly and patients with basic diseases can develop a more serious condition after infection, but there are less cases of COVID-19 in children and infants”.

Based on the fourth edition, the source of infection has been changed to that“at present, the major source of infection is the patients with COVID-19, and asymptomatic 2019-nCoV carriers can also be the source of infection” in the fifth edition. The descriptions of transmission routes have been changed to that“COVID-19 is mainly transmitted by respiratory droplets and contact, and routes such as aerosol and gastrointestinal transmission remain to be confirmed”. The description of susceptible groups has also been changed to that “Humans of all ages are generally susceptible”.

The source of infection in the sixth edition is changed from “COVID-19 is mainly transmitted by respiratory droplets and contact” to “COVID-19 is mainly transmitted by respiratory droplets and close contact”, in other words, “close” has been added in front of “contact”. Besides, “Aerosol transmission is possible when humans have prolonged exposure to high concentration of aerosol in a relatively closed space” is added in the sixth edition.