Aggressive measures work.

New cases have dropped to 200 a day from over 3,000 a day one month ago. After the initial chaos and cover-up in Wuhan, health authorities imposed a lockdown, strict quarantines, mandatory testing and isolation. That prevented what would have been hundreds of thousands of infections.

There aren’t many asymptomatic cases.

Testing of 320,000 samples suggests that the known cases are not just the tip of an iceberg. “What we’re seeing is a pyramid: Most of it is above ground,” Dr. Aylward said.