The surprise statement from Prof Shi Zhengli comes as a World Health Organization team prepares to travel to Wuhan next month to begin its investigation into the origins of Covid-19.
The remote district of Tongguan, in China's south-western province of Yunnan, is hard to reach at the best of times. But when a BBC team tried to visit recently, it was impossible.
Plain-clothes police officers and other officials in unmarked cars followed us for miles along the narrow, bumpy roads, stopping when we did, backtracking with us when we were forced to turn around.
