Adding infections, subtracting deaths: What to make of China’s fluctuating coronavirus count

There are pros and cons to this week's wild swings in data.

For anyone keeping close track of China’s coronavirus counts this week, the numbers may have been equal parts concerning and confusing.

On Thursday, the Chinese city of Wuhan, the epicenter of the outbreak, reported a record 15,000 new cases of coronavirus, known as COVID-19, from the previous day. Wuhan authorities said the uptick reflected an effort to incorporate probable cases of the virus that weren’t necessarily confirmed in a laboratory so as to help medical professionals treat more patients.

Subscribe to unlock this article and get full access to Fortune.com

For anyone keeping close track of China’s coronavirus counts this week, the numbers may have been equal parts concerning and confusing.

On Thursday, the Chinese city of Wuhan, the epicenter of the outbreak, reported a record 15,000 new cases of coronavirus, known as COVID-19, from the previous day. Wuhan authorities said the uptick reflected an effort to incorporate probable cases of the virus that weren’t necessarily confirmed in a laboratory so as to help medical professionals treat more patients.

On Friday, an additional 5,000 new infected patients were discovered, bringing the total number of cases reported worldwide to over 64,000. Also on Friday, Wuhan reported 121 new deaths due to the disease while subtracting 108 from the previous total, saying that some deaths had previously been double-counted. The total number of deaths related to coronavirus is now approaching 1,400.

Given low domestic and international trust in Wuhan’s handling of the early stages of the outbreak, China’s data about the coronavirus has given some experts pause; they argue that China should have been more forthcoming with data from the beginning. If nothing else, Thursday’s new numbers “undermine [the] message” that the central government is in control of the situation, Chinese politics professor Sam Crane of Williams College told The Guardian.

Though the change in methodology and subsequent spike in cases may be frustrating to some observers, it gives a “better reflection of the current situation” rather than signifying a huge increase in actual cases, said Martin Chan, a professor at Stanley Ho Centre for Emerging Infectious Diseases at the University of Hong Kong. “The sheer volume of suspected cases at the epicenter” has created a “huge backlog” in testing kits and ability to make lab reports, he said.

What’s more, shifting figures are to be expected, given the “emergency nature of outbreak response,” said microbiology professor Ooi Eng Eong of Duke-NUS Medical School in Singapore.

Projection problems

And the big upside of the methodological change is that patients who haven’t been officially diagnosed with coronavirus might receive better care, said to Benjamin Cowling, infectious disease expert and professor at Hong Kong University’s School of Public Health.

“In outbreaks it is not possible to test every single suspected case or even every single probable case, and for clinical management it is usually sufficient to use the same drugs and the same care strategies for ‘probable cases’ as for ‘confirmed cases,’” said Cowling.

Indeed, there have been ample anecdotal reports of suspected cases in Wuhan that have been left off official tallies, and the new methodology may provide some hope that more of those infected will receive treatment.

Nevertheless, the sudden change in how Wuhan is measuring the outbreak will make it difficult to project the outbreak’s growth in the short term.

“The change in case definition now makes it difficult to compare the number of cases before [it went into effect] with those that are reported after,” said Ooi. And it will make interpreting disease trends “tricky” over the next few days. Gathering significant takeaways from data takes time, Ooi said, so it will be a few weeks before any meaning can be derived from the data reported under the new definition.

Data alternatives

Even before this week, experts had been hunting for alternative sources of data. The situation in Wuhan is so dire, with health professionals focused on patient care, that they knew collecting perfect data from the epicenter would be nearly impossible.

“Experts have been looking at different sources of outbreak data and considering different possible scenarios based on them,” said Eugene Hung, a life science professor at Hong Kong University of Science and Technology. Hung, for his part, has been evaluating the number of cases in people who have left Wuhan’s Hubei province since the outbreak, and found that roughly 1% of them have been infected. Using that model to account for the whole province would put the actual number of confirmed cases in the “hundreds of thousands.”

Infectious disease expert Neil Ferguson at Imperial College London has estimated that only about 10% of those infected in China have likely been detected, and there may be as many as 50,000 new cases per day.

Yet if there’s any encouragement in projections related to the coronavirus, it’s that the vast majority of the cases appear to be mild.

Chan said that the death-to-case ratio seems to be holding strong at around 2%.

“At least this ratio, which is important, appears to be reliable and that is good,” Chan said.

More must-read stories from Fortune:

—Can you catch it twice? Answers to 5 pressing coronavirus questions
Why China is still so susceptible to disease outbreaks
—Bernard Arnault was briefly the world’s richest man. Then coronavirus struck
Will summer kill the coronavirus?
—My boss wants me to travel during the coronavirus. Do I have to go?

Subscribe to Fortune’s Brainstorm Health for daily updates on biopharma and health care.