How the virus got out

Many of the first known cases of the coronavirus clustered around a seafood market in Wuhan, China, a city of 11 million and a transportation hub.

It wasn’t until about one month in that China notified the World Health Organization — and reassured it there was nothing to worry about. But it was already late — hundreds of millions of people at the time were going to travel for the Lunar New Year.

Thousands of travelers left Wuhan to go to other major cities, and about 85 percent of those infected went undetected, researchers estimated.

Our data journalists analyzed the movements of hundreds of millions of people to show why the most extensive travel restrictions in human history haven’t been enough to stop the coronavirus outbreak.

Takeaway: Even a perfect response may not have contained the spread — but there were holes and gaps in the global response that are clearer now. By the time Chinese officials acknowledged the risk of human-to-human transmission on Jan. 21, local outbreaks were already seeded in major cities. And international travel continued as usual for several weeks.