New evidence from China states what epidemiologists have long suspected: the coronavirus probably began to spread unnoticed in the Wuhan area in November 2019, before exploding in several different locations in the city in December.
Chinese authorities have identified 174 confirmed cases of Covid-19 in the city as of December 2019, according to World Health Organization researchers, enough to suggest that there were many more mild, asymptomatic or undetected cases. than previously thought.
Many of the 174 cases had no known connection to the market that was initially considered the source of the outbreak, according to information gathered by WHO researchers during the four-week mission to China to examine the origins of the virus. Chinese authorities refused to provide the WHO team with raw data on these and possible previous cases, team members said.
Examining 13 genetic sequences of the virus as of December, Chinese authorities found similar sequences among those related to the market, but slight differences in those of unrelated people, according to WHO researchers. The two sets probably began to diverge between mid-November and early December, but could possibly indicate infections as early as September, said Marion Koopmans, a Dutch virologist on the WHO team.
This, and other evidence, suggest that the coronavirus may have jumped on humans sometime during or shortly before the second half of November, he said, sickening too few people to draw attention to until it caused an explosive outbreak in Wuhan. In December, the virus spread much more, both among people who had a link to the market and to other people without ties.