On Apr. 23, 2020, a study led by researchers at Oxford and Guangdong Centre for Diseases Control and Prevention described the epidemiology and genetic make-up of the COVID-19 outbreak in Chinaﾒs most populous region, and showed how early and intensive testing and tracing helped to interrupt local transmission of the virus. The study combined genome sequencing and epidemiology to reveal three phases of the outbreak in Guangdong Province, China.