Despite a high-profile visit to China by a team of international experts in January, the world is no closer to knowing the origins of Covid-19, according to one of the authors of an open letter calling for a new investigation into the pandemic.

"At this point, we are no further advanced than we were a year ago," said Nikolai Petrovsky, an expert in vaccines at Flinders University in Adelaide, Australia, and one of 26 global experts who signed the open letter, published on Thursday.

In January, a team of scientists picked by the World Health Organisation (WHO) visited hospitals and research institutes in Wuhan, the central Chinese city...