Epidemiologists are disease detectives. The investigative insights of a forensic epidemiologist are exemplified by Sherlock Holmes, whose creator, Arthur Conan Doyle, qualified as a medical doctor in Edinburgh. With limited information, some of which may be dubious and misleading, epidemiologists search for hidden clues as to the cause of a disease and its manner of population spread and use statistical modeling techniques to estimate the degree of disease contagion and the number of cases of infection.
Prof. Neil Ferguson heads the World Health Organization (WHO) Collaborating Center for Infectious Disease Modeling at Imperial College London. His search for scientific understanding using sparse observational data dates back to his theoretical physics PhD at Oxford. Like others trained in theoretical physics, Prof. Ferguson is not shy in making mathematical forecasts that may be at odds with partial data of suspect reliability. Misreporting blighted the Chinese response to the 2002 SARS outbreak.
As of January 16, 2020, 41 coronavirus [2019-nCoV] cases had been confirmed in Wuhan, with just three confirmed cases outside China: two in Thailand and one in Japan. The two Thai cases traveled from Wuhan on January 8 and 13; the Japanese case traveled from Wuhan on January 6.
A simple back-of-the-envelope calculation shows that the number of reported cases in China must have been understated by at least one order of magnitude. If there were only a few dozen cases in Wuhan, it would be surprising that there were as many as three cases abroad. Each day there were about 3,000 international passengers flying out of Wuhan airport.
For the early 2020 time window from January 4-13, three out of about 30,000 international passengers, i.e. 1/10,000 were confirmed with the novel coronavirus. Given a Wuhan catchment population of 10 million or more, the number of cases in Wuhan should be at least one thousand. This would have come as a disappointment to optimists who might have hoped that 2019-nCoV would be rapidly controlled within China.
As of Friday, January 24, 2020, over 900 cases with 26 deaths have been officially reported in China, with cases detected in at least nine regions outside mainland China. Three days earlier, Prof. Ferguson and colleagues estimated 4,000 cases in Wuhan. Tracking the evolution of the outbreak, they have made the first assessment of the transmissibility of the novel coronavirus [pandemic/Imperial-2019-nCoV-tranmissibility].
They estimate 2.6 for the reproduction number, R0, which measures the average number of new infections generated by each infected person. Draconian control measures, such as the lockdown of cities in Hubei province and the closure of popular tourist destinations, have been being introduced in China to lower R0, but the timing of the Chinese New Year mass migration is highly unfavorable for voluntary self-isolation.
A challenge for public health officials is identifying and testing suspected cases with mild influenza-like symptoms. Fortunately, the case fatality rate seems to be comparatively low for a coronavirus: about three percent compared with 10 percent for SARS and 35 percent for MERS. Infectious disease risk stakeholders around the world will hope that, as the coronavirus spreads, it does not mutate into a more deadly infection.