Early transmission dynamics of the novel coronavirus in Wuhan, China

1. Analysis of the initial transmission dynamics of the novel corononvirus revealed the epidemic to be doubling in size approximately every week with each infected individual having transmitted the disease to 2.2 other persons on average.

2. Mean incubation period was estimated to be 5.2 days, with the 95^th percentile of distribution being 12.5 days, supporting a 14-day medical observation or quarantine period for exposed persons.

Evidence Rating: 2 (Good)

Study Rundown: The novel coronavirus, or 2019-nCoV, represents one of the largest epidemic outbreaks in recent memory, with nearly 100,000 cases have been reported worldwide in over 70 countries. In this study, initial cases of 2019-nCoV were analysed to provide valuable epidemiological information regarding the virus’s early transmission dynamics into becoming an epidemic. Analysis of the first 425 confirmed cases of confirmed 2019-nCoV –infected pneumonia revealed a greater proportion of male versus female patients (56%), and few cases to have affected children, with the median age of patients to be 59 years (range, 15 to 89).

While zoonotic and environmental exposures remain possible routes of transmission, the study strongly reinforced human-to-human transmission to have been occurring and as a primary source of epidemic development. Specifically, it was estimated that each patient was on average spreading the disease to 2.2 other people, contributing to a doubling rate of 7.4 days. Using the data among 10 confirmed cases, the mean incubation period was estimated to be 5.2 days, with the 95^th percentile of distribution being 12.5 days, supporting 2 week observation or quarantine period for exposed individuals. Finally, mean duration from illness onset to medical visit was estimated to be 5.8 days, and mean duration to admission 12.5 days. While the study’s inclusion criteria to focus on more severe cases of the virus (i.e., those requiring significant medical attention) limits the ability of the study to fully characterize transmission dynamics, this study provides invaluable data for further analysis and represents an important step in further outbreak prevention.

Click to read the study in NEJM

Relevant Reading: Characteristics of and Important Lessons From the Coronavirus Disease 2019 (COVID-19) Outbreak in China

In-depth [retrospective cohort]: The study collected data via field reports, field investigations, and standardized interviews of patients, contacts, and health care providers related to 425 cases with specimens confirmed in laboratory. Interviews were conducted to characterize exposure histories during the 2 weeks before illness onset, including environmental and zoological exposures. Case definition for study inclusion entailed a case with respiratory specimens testing positive for the virus via either isolation of 2019-nCoV or 2 positive results in RT-PCR for 2019-nCoV. Epidemic growth rate was estimated via analyses of cases with onset between December 10, 2019 and January 4, 2020,  fitting a transmission model with zoonotic infections that were not linked to the Huanan Seafood Wholesale Market. From this, growth rate, doubling rate, and basic reproductive number were estimated. Incubation distribution was estimated by fitting a log-normal distribution to data on exposure histories and onset dates. Epidemic growth rate up to January 4, 2020 was estimated to be 0.10 per day (95% CI, 0.050 to 0.16), of which the doubling time (7.4, 95% CI, 4.2 to 14) and reproductive number (2.2, 95% CI, 1.4 to 3.9) were derived. The mean incubation period was estimated to be 5.2 days (95% CI, 4.1 to 7.0), with the 95^th percentile of distribution being 12.5 days (95% CI, 9.2 to 18).  Finally, mean duration from illness onset to medical contact was 5.8 days (95% CI, 4.3 to 7.5), and duration from onset to admission 12.5 days (95% CI, 10.3 to 14.8)

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