BMJ 2020 ; 371 doi: https://doi.org/10.1136/bmj.m4695 (Published 01 December 2020) Cite this as: BMJ 2020;371:m4695 A mass screening programme of more than 10 million residents of Wuhan, China, performed after SARS-CoV-2 was brought under control, has identified 300 asymptomatic cases of covid-19, none of which was infectious.