There are now so few coronavirus cases in China that some days, authorities don't see any local transmission. China has gone from reporting thousands of cases a day in February to reporting one or two a day now. Over the past week, officials with China's National Health Commission reported just five new domestic cases. The total of new cases was higher, but almost all of them were imported cases in travelers who had recently returned from abroad.

China has driven coronavirus transmission down to nearly zero. (Although there's some question among international academics about China's case reporting and whether some cases are being overlooked, it's generally agreed that the nation has suppressed transmission to a very low level.) But some undetected cases are still probably floating around, and the virus can always be brought back in from abroad.

Ben Cowling, an epidemiologist at the University of Hong Kong, says these "imported" cases are much easier to contain and don't pose as much of a threat of wider transmission.

"There's less opportunity for infections to get into the general community from those travelers because they are being monitored so carefully," Cowling says. Anyone arriving in China must go into 14 days of quarantine where officials can keep a close eye on their health. "So if they do turn out to be infected, which is a small minority, then they're isolated. Their contacts are traced and put into quarantine. And that's going to slow down any any leakage into the general community."

Cowling says public health officials in China and elsewhere have two sets of tools to contain its outbreak — social distancing and case management. Social distancing makes it harder for the virus to find new people to infect. Case management tracks down cases and potential cases individually and then isolates them – and the virus. China wielded both of these tools aggressively.