How the Coronavirus Numbers Changed So Sharply

“We need to be careful in interpreting these numbers,” Dr. Michael Ryan, the World Health Organization’s emergency responses chief, said on Thursday. The larger number of cases, he said, “does not represent a significant change in the trajectory of the outbreak.”

The W.H.O. then released its own figures, leaving out the diagnoses made without a positive test for the virus — and indicating that it did not fully understand the change China had made.

“For consistency, we report here only the number of laboratory-confirmed cases,” the organization said in a situation report. “W.H.O. has formally requested additional information on the clinically diagnosed cases, in particular when these have occurred in the course of the outbreak and whether suspect cases were reclassified as clinically diagnosed cases.”

The W.H.O. reported almost 47,000 confirmed cases worldwide, fewer than 1 percent outside mainland China. The Chinese figures put the total at more than 61,000 cases.

What was wrong with the old way of counting?

In a word, plenty.

Kits to test for the virus are not being produced quickly enough to keep up with demand, and they take days to produce results. Using CT scans means more patients can be checked, with the results available almost immediately.

And the tests for the virus have not been very reliable. Many sick people have tested negative, only to test positive later. That appears to be, at least in part, because standard testing involves swabbing nasal passages and the throat, while the virus may be lodged in the lungs.