Shanghai's unusually high proportion of asymptomatic cases among its reported Covid-19 infections could be partly explained by blurred lines between "mildly symptomatic" and asymptomatic cases.

Authorities in China's financial centre ordered a lockdown in response to the worst outbreak in the country since the virus first emerged in Wuhan in late 2019.

Shanghai counted more than 20,000 new cases on April 7, but the asymptomatic rate has stood at around 97 percent, far higher than anywhere else in the world, where it has been closer to 50 percent.

Fu Chen, head of the municipal Center for Disease Control and Prevention, said in a written reply to Reuters' questions that...