Study population and setting
This study included the first 425 laboratory confirmed cases of novel coronavirus (2019-nCoV)–infected pneumonia (NCIP) between December 2019 to January 2020. A confirmed case was defined as a case with respiratory specimens that tested positive for the 2019-nCoV by at least one of the following three methods: isolation of 2019-nCoV; or at least two positive results by real-time reverse-transcription–polymerase chain- reaction (RT-PCR) assay for 2019-nCoV; or a genetic sequence that matches 2019-nCoV. Data were collected through interviews of infected persons, relatives, close contacts, and health care workers and included information on the dates of illness onset, visits to clinical facilities, hospitalization, and clinical outcomes; epidemiologic data were collected through interviews and field reports.
Summary of Main Findings
The mean incubation period was estimated to be 5.2 days (95% confidence [CI], 4.1 to 7.0). The serial interval distribution had a mean (±SD) of 7.5±3.4 days (95% CI, 5.3 to 19) and the basic reproductive number (R0) was estimated to be 2.2 (95% CI, 1.4 to 3.9). Based on these parameters the epidemic growth rate was 0.10 per day (95% CI, 0.050 to 0.16) and the doubling time was 7.4 days (95% CI, 4.2 to 14).
This study utilized the first confirmed cases of COVID-19 to provide early estimations for natural history and transmission dynamics.
Cases comprised of individuals with severe enough illness to require medical attention, which may vary according to the presence of coexisting conditions, biasing results to more severe cases and lacking representation from mild or asymptomatic cases. Additionally, the use and availability of testing kits progressed over time and may have introduced bias.
These data from early in the epidemic provided important measures for characterizing the natural history and transmission dynamics including estimates of incubation period, serial interval, basic reproductive number, epidemic growth rate, and doubling time.