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 US 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...