China reported the highest number of asymptomatic Covid-19 cases in nearly seven months, following a mass infection of an unknown origin in the northwestern Xinjiang region.

Xinjiang health authorities found 137 asymptomatic cases on Sunday amid a testing drive for the 4.75 million people in the Kashgar area triggered by an asymptomatic infection in a 17-year-old female garment factory worker reported on Saturday. It was not clear how she was infected, though all of the new cases were linked to the garment factory...