There’s a lot we don’t yet know about the Wuhan coronavirus (2019 nCoV, later renamed COVID-19), but we do know that it is spreading faster than the 2003 outbreak of SARS.

According to CNN, the 2003 SARS outbreak infected 5,327 people in mainland China and resulted in 349 deaths. Those numbers occurred over nine months.  In contrast, the death toll from the Wuhan coronavirus has topped 425 with more than 20,000 confirmed cases in China as of February 4^th. The outbreak was first reported to the World Health Organization (WHO) on December 31^st, a mere five weeks ago.

Medical workers transporting a coronavirus patient into an isolation ward in China. Credit: The New York Times/Chinatopix, via Associated Press

Scientists around the globe are studying the outbreak and trying to understand the best way to contain the spread and head off a global pandemic. Here’s a look at one such study.

Tracking early transmission dynamics

A study, published last week in the New England Journal of Medicine, analyzed the data from the first 425 confirmed COVID-19 cases in Wuhan. Researchers from the Chinese Center for Disease Control and Prevention and the WHO collaborated with scientists across China for this study.

The teams cross-checked and analyzed epidemiologic information collected during field investigations, including exposure history, timelines of events, and close contact identification. The team performed a statistical analysis and constructed the epidemic curve by date of illness onset, estimating the incubation period based on exposure histories and onset dates of subsequent cases.

The timeline for these 425 cases is shown below.

Image credit: The New England Journal of Medicine

The onset of illness among the first 425 confirmed cases of 2019 nCoV. The decline in incidence after January 8th is likely due to delays in diagnosis and laboratory confirmation. Image Credit: The New England Journal of Medicine

The information enabled them to estimate the epidemic growth rate. Analyses of the incubation period, serial interval, growth rate, and R₀  were performed with MATLAB. R₀  is defined as the expected number of additional cases that one case will generate, on average, over the course of its infectious period in an otherwise uninfected population.

This analysis enabled the team to estimate the COVID-19 mean incubation period of 5.2 days. They also calculated the time to first medical visit and hospital admission. Those results are shown below.

(A) The estimated incubation period. (B) The estimated serial interval distribution. (C) The estimated distributions of times from illness onset to the first medical visit. (D) The estimated distributions of times from illness onset to hospital admission. Image Credit: The New England Journal of Medicine

Study shows human-to-human transmission from the start

The study authors concluded, “On the basis of this information, there is evidence that human-to-human transmission has occurred among close contacts since the middle of December 2019. Considerable efforts to reduce transmission will be required to control outbreaks […]”

The study found that each infected patient, on average, has been spreading the infection to 2.2 other people. This indicates the importance of isolating patients early in the illness onset. An R₀ of 2.2 shows the epidemic is growing.  In general, an epidemic will increase as long as R₀ is greater than 1.

The researchers also flagged the delay between first visit to medical professionals and the time of hospitalization, stating, “It may be necessary to commit considerable resources to testing in outpatient clinics and emergency departments for proactive case finding, both as part of the containment strategy in locations without local spread yet as well as to permit earlier clinical management of cases.

“In conclusion, we found that cases [of COVID-19] have been doubling in size approximately every 7.4 days in Wuhan at this stage. Human-to-human transmission among close contacts has occurred since the middle of December and spread out gradually within a month after that.

“Urgent next steps include identifying the most effective control measures to reduce transmission in the community.”