SINGAPORE, Nov 19 (Reuters) - The first known COVID-19 case was a market vendor in the Chinese city of Wuhan, not an accountant who appeared to have no link to the market but whose case contributed to speculation the virus could have leaked from a lab, according to a U.S. study. The origin of the SARS-CoV-2 virus that causes COVID-19 remains a mystery and a major source of tension between China and the United States. A joint study by China and the World Health Organization (WHO) this year all but ruled out the theory that COVID-19 originated in a laboratory, saying that the most likely hypothesis was that it infected humans naturally, probably via the wildlife trade. A WHO-led team of experts spent four weeks in and around the central city of Wuhan with Chinese scientists and said in a joint report in March that the SARS-CoV-2 virus had probably been transmitted from bats to humans through another animal but that further research was needed.The accountant, who was widely thought to be the first person with COVID-19, reported that his first symptoms appeared on Dec. 16, several days later than initially known, Michael Worobey, head of ecology and evolutionary biology at the University of Arizona, said in the study published in the journal Science on Thursday. The confusion was caused by a dental problem he had on Dec. 8. "His symptom onset came after multiple cases in workers at Huanan Market, making a female seafood vendor there the earliest known case, with illness onset 11 December," the study said. It said most early symptomatic cases were linked to the market, specifically to the western section where raccoon dogs were caged, and it provided strong evidence of a live-animal market origin of the pandemic. The WHO proposed last month a new expert panel to investigate the source of the coronavirus.