Human-to-human transmission of the coronavirus was occurring from mid-December 2019 in Wuhan, China, according to the largest study to-date analyzing patients infected with the deadly virus.
A new paper, published in the New England Journal of Medicine on Jan. 29, examined data from the first 425 confirmed cases in the central Chinese city of Wuhan, and found that “there is evidence that human-to-human transmission has occurred among close contacts since the middle of December 2019.”
Chinese authorities did not confirm human-to-human transmission until Jan. 20, almost three weeks after the disease was first officially reported on Dec. 31, 2019. The first patient exhibited symptoms on Dec. 1.
Official figures report thousands being infected in China and more than a hundred killed—though experts say the actual number infected is much higher.
The World Health Organization (WHO) on Jan. 30 declared the virus a global health emergency after the disease spread to 18 countries outside of China with 98 confirmed cases.
At least four cases of human-to-human transmission—that is, cases of people who haven’t been to China but contracted the disease from someone who had—have been reported overseas. The U.S. Centers for Disease Control and Prevention on Jan. 30 confirmed that an Illinois man contracted the virus from his wife, who had recently returned from Wuhan. Other such cases have been identified in Japan, Taiwan, and Germany.
The study was conducted by a team of researchers, including those affiliated with China’s Center for Disease Control and Prevention in Beijing and the Hubei Provincial Center for Disease Control and Prevention. Wuhan is the capital of Hubei Province.
The researchers found that the outbreak doubled in size every 7.4 days during the initial stages.
They also estimated that each patient then infected an average of 2.2 other people, a figure known as the basic reproductive number. This makes the coronavirus comparable to the SARS (severe acute respiratory syndrome) outbreak of 2002 to 2003, which was estimated to have a reproductive number of 3.
The number of officially reported coronavirus infections in China—just one month after the initial outbreak—has now surpassed the total number of SARS infections reported by authorities then.
The study estimated that the virus’s average incubation period—that is, the period between exposure and the onset of symptoms—was 5.2 days. However, they noted that the incubation period varied greatly among patients. The WHO said on Monday that the incubation period ranged between two to 10 days, before symptoms like fever, cough, shortness of breath, and acute respiratory distress appeared.
The median age of the patients was 59, although they ranged from 15 to 89; 56 percent of the patients were men.