New evidence from China confirms what epidemiologists have long suspected: The coronavirus likely started to spread unnoticed around the Wuhan area in November 2019, before exploding in several different locations around the city in December.
Chinese authorities have identified 174 confirmed cases of Covid-19 in the city as of December 2019, researchers from the World Health Organization said, enough to suggest that there were many more mild, asymptomatic, or otherwise undetected cases than previously thought.
Many of the 174 cases were not known to be related to the market initially believed to be the source of the outbreak, according to information gathered by WHO researchers during the four-week mission to China to investigate the origin of the virus. Chinese authorities refused to provide the WHO team with raw data on these cases and possible previous cases, team members said.
When examining 13 genetic sequences of the December virus, the Chinese authorities found similar sequences among the sequences linked to the market, but small differences in those of people without any link, WHO researchers said. The two sets likely began to diverge between mid-November and early December, but could potentially indicate infections as early as September, said Marion Koopmans, a Dutch virologist on the WHO team.
This, and other evidence, suggests the coronavirus may have hit people sometime during or shortly before the second half of November, she said, making too few people sick to draw attention until it sparked an explosive outbreak in Wuhan. In December, the virus spread much more widely, both among people with a link with the market and among others without a link.