Medical personnel and patients with mild symptoms of Covid-19 rest at night in a temporary hospital set up in a sports stadium in Wuhan, China.

Infectious diseases experts are losing confidence in the accuracy of China’s count of cases of the novel coronavirus, pointing toward health officials’ shifting definition of cases over time.

Confusion over how China is counting cases of infections is making it harder to know how coronavirus is spreading, even as China is officially reporting that the numbers of new cases reported in recent days have fallen sharply. Many suspect the decline may be attributed in part to shifting case definitions. Earlier this month, China broadened the criteria for newly diagnosed cases in Hubei province, the epicenter of the outbreak, then reversed itself.

“Every time you change the case definition, that then means you have a reset in terms of what you’re actually looking at,” said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy. “I think between the inability to determine the actual number of people infected and how cases are now being called a case means at best you can get trend data, possibly, but not more than that.”

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China reported 74,675 cases and 2,121 deaths to the World Health Organization as of Thursday morning, the agency’s director general, Tedros Adhanom Ghebreyesus, said during a daily briefing for reporters. Another 1,076 cases have been reported from 26 other countries, including seven deaths.

From a few dozen cases in early January, China’s daily increase in cases exploded later in the month. The rapid increase of cases in the lead-up to the Lunar New Year led Chinese authorities to impose a quarantine of the entire city of Wuhan, where the outbreak appears to have started, on Jan. 22.

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In the days that followed, the quarantine extended to other cities in Hubei province, leaving tens of millions of people under the largest “social distancing” effort of modern times. And still the country’s case count for Covid-19, the disease caused by the coronavirus, rose by between 2,500 and 3,400 cases a day.

On Feb. 12, the national health committee announced that it was changing the way cases were counted in Hubei province. People with symptoms of the disease and with evidence of pneumonia on a CT scan but who had not been tested for presence of the virus — in other words, people who had a clinical diagnosis — would be added to the list of cases.

That day, the country reported a massive increase in cases — over 15,000. But since then, the daily numbers have tumbled. On Wednesday, China announced it was reverting to reporting laboratory-confirmed cases only. When the National Health Committee issued its update for Wednesday, it reported a net increase of only 394 cases — the first time in weeks the daily case increase was under 1,000 cases.

Helen Branswell/STAT

It is not unusual for the case definition to change during an infectious diseases outbreak caused by a new virus. In fact, they have to change as more information becomes apparent. For too long in this outbreak, for example, China only counted people with pneumonia as cases — a fact that blocked their ability to detect people with mild infections. But the relatively rapid about-face on the inclusion of clinically diagnosed cases has amped up the uncertainty about the published numbers.

Caitlin Rivers, an assistant professor of epidemiology at the Johns Hopkins Center for Health Security, called the situation confusing. When she spotted that China was reversing its approaching to clinical diagnosis, she assumed the number of suspected cases would rise as a result. But it didn’t increase measurably.

Rivers said from her perspective, there are two possibilities — the draconian social distancing measures China has taken are really driving down the number of cases — conceivably, she said — or health officials are changing their view of what constitutes a case.

Coronavirus expert Ralph Baric, from the University of North Carolina, is uneasy about the numbers coming out of China.

“I’m very suspicious about anything they’re saying,” Baric said, pointing to the low numbers China is reporting from other provinces in the country. “The math says there should be a lot more cases.”

The single largest obstacle in trying to assess the size of the outbreak in China is the fact that the way the country has been looking for cases makes it almost impossible to log the large number of mild infections experts believe are occurring.

That belief is based on the type of disease recorded in people who contracted the disease in China but who were diagnosed in other countries. While there are people who are severely ill among the infections detected outside China, many have very mild, even cold-like symptoms.

In the midst of a crisis like the one that has struck Wuhan, finding mild infections requires health authorities to look beyond the people who show up at hospitals for care and do what is called a serological survey.

That involves drawing blood from a representative sample of people — making sure you draw from the full range of age groups — to test for antibodies to the virus. Baric, who has collaborations with scientists in Wuhan and has seen the sophistication of the research facilities there, feels certain that work would already have been done.

A serological survey would fill a critical information gap, said Malik Peiris, a virologist at Hong Kong University who is one of the leaders in the response to the 2003 SARS outbreak. (SARS is closely related to the virus that causes Covid-19.)

“I think the one single most important thing China can do now … to reassure the population in China, the population in the world about this outbreak, is to do exactly this type of sero-epidemiological study in Wuhan or somewhere in Hubei,” Peiris told STAT.

If, as experts expect, that kind of study showed that many mild cases are escaping detection, the world would have a better grasp of how severe this outbreak is and what it would need to expect should the virus sweep the globe.

“This is the only way we can get a true estimate of the actual severity, the actual mortality of this disease,” Peiris said.