https://www.sciencedaily.com/releases/2020/03/200331130012.htm

"The number of confirmed cases in China by day 50 (February 19) of the epidemic, was around 30,000," said Christopher Dye, visiting professor of zoology and visiting fellow at the Oxford Martin School, University of Oxford. "Our analysis suggests that without the Wuhan travel ban and the national emergency response there would have been more than 700,000 confirmed COVID-19 cases outside of Wuhan by that date. China's control measures appear to have worked by successfully breaking the chain of transmission -- preventing contact between infectious and susceptible people."

The researchers used a unique combination of case reports, human movement data and public health intervention information to investigate the spread and control of COVID-19. They examined the movements of 4.3 million people out of Wuhan before the travel ban, the types and timing of control measures implemented across the cities of China and the numbers of COVID-19 cases reported each day in every city.