New evidence from China is confirming what epidemiologists had long suspected: the coronavirus probably started to spread unnoticed in the Wuhan area in November 2019, before it exploded in several different locations in the city in December.
Chinese authorities identified 174 confirmed cases of Covid-19 in the city in December 2019, researchers from the World Health Organization said, 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 to be the source of the outbreak, according to information collected by WHO researchers during the four-week mission to China to examine the origins of the virus. Chinese officials have refused to provide WHO staff with raw data on these cases and possible past cases, team members said.
In examining 13 genetic sequences of the December virus, Chinese officials found similar sequences among those linked to the market, but slight differences in those of people with no connection to it, according to WHO researchers. The two sets probably started to diverge between mid-November and early December, but may 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 hit humans sometime during or shortly before the second half of November, she said, leaving very few people sick to draw attention until it led to an explosive outbreak in Wuhan. In December, the virus was spreading much more widely, both among people who were linked to the market and among others who were not.