A joint China-World Health Organization (WHO) study into Covid-19 has provided no credible answers about how the pandemic began, and more rigorous investigations are required - with or without Beijing's involvement, a group of international scientists and researchers said on Wednesday.

The joint study, released last week, said the likeliest transmission route for SARS-CoV-2, the virus that causes Covid-19, involved bats and other wildlife in China and Southeast Asia. It all but ruled out the possibility it had leaked from a laboratory...