Initial outbreaks of COVID-19 in China found that 13.8% cases were severe and 6.1% were critical. This severe presentation seems to be because virus receptor is expressed predominantly in the lung.

This study describes a detailed virological analysis of nine cases, providing proof that the virus actively replicates in upper respiratory tract tissues. Pharyngeal virus shedding is very high in the first week of symptoms. Infectious virus was isolated from the throat and lungs easily, but not from stool. Blood and urine samples never yielded virus.

The shedding of viral RNA from sputum outlasted the symptoms. Seroconversion occurred after 7 days in half of patients, but was not followed by a rapid decline in viral load.