Previously overlooked Chinese data on extensive screening of animals for coronavirus around the time the pandemic erupted is among several areas identified for further study by World Health Organization (WHO) scientists investigating the origins of Covid-19, a source close to the team told CNN.

The source said the records are contained in a nearly 200-page annex posted alongside the WHO panel's March report that received little attention among global experts at the time. But the data may add weight to calls from China's critics for more transparency and to the WHO team's desire to return to the country for further studies.

No date has been set for the team's return to China, but the source said any future visit to the country — where the virus emerged in the city of Wuhan, Hubei province, in late 2019 — may involve “smaller groups supporting specific studies first.” A larger group, similar to the 17 international experts that visited in January, might then follow up, the source added.

The WHO report's annex contains multiple data points providing an intriguing insight into China's evolving knowledge of the virus and the likely timing of its emergence.

It provides details of China's storage and destruction of positive Covid-19 samples from humans; a significant influenza outbreak that emerged in December 2019, at the same time as the virus; and the revelation that the first people known to have contracted the virus had contact with a total of 28 separate food and animal markets in December.