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 Covid-19 cases around the city since December 2019, World Health Organization researchers said, enough to suggest that there were more mild, asymptomatic or otherwise undetected cases than previously thought.
Many of the 174 cases had no known link to the market, which was originally considered the source of the outbreak, according to information gathered by WHO investigators during the four-week mission to China to examine the origins of the virus. Chinese authorities have refused to provide the WHO team with raw data on these and potential previous cases, team members said.
In examining 13 genetic sequences of the virus in December, Chinese authorities found similar sequences among those related to the market, but slightly different from those unrelated to it, according to WHO investigators. The two sets probably began to diverge between mid-November and early December, but could indicate infections as early as September, said Marion Koopmans, a Dutch virologist on the WHO team.
This and other evidence suggests that the coronavirus may have jumped on humans sometime or shortly before the second half of November, she said, making too few people sick to attract attention until it led to an explosive outbreak in Wuhan. By December, the virus had spread much more, both to people who had a connection to the market and to others without ties.