Some five million people have already left Wuhan, the epicenter of the outbreak — presenting challenges about how to tackle the epidemic.

“It’s definitely too late,” said Jin Dong-Yan, a molecular virologist at Hong Kong University’s School of Biomedical Sciences. “Five million out. That’s a big challenge. Many of them may not come back to Wuhan but hang around somewhere else.”

Jin said that officials need to find a way to locate travelers who have left Wuhan.