HONG KONG — The news was abrupt and, to some, surprising: Overnight, a Chinese province near Russia, had cut its count of confirmed coronavirus cases by more than a dozen.
The revision stemmed from what appeared to be a bureaucratic decision, buried in a series of dense documents from the national government. Health officials said that they would reclassify patients who had tested positive for the new coronavirus but did not have symptoms, and take them out of the total count of confirmed cases.
The documents offered little detail or explanation, and skepticism was immediate. A Hong Kong newspaper called the decision a “disguise.” World Health Organization officials seemed caught off guard when asked about the move at a news conference this week.
The change in counting cases is only one factor that has made it difficult for experts to determine the true scale of the epidemic. In fact, the shifting landscape of how infections are defined and confirmed has led to significant variations in the estimates for the extent of outbreak.
Early on Thursday, provincial officials in Hubei province, the center of the outbreak, announced that nearly 15,000 new cases and 242 new deaths were recorded in a single day, largely because the authorities expanded their diagnostic tools for counting new infections.
Until now, only infections confirmed by specialized testing kits were considered accurate. But those kits have been in such short supply — and so many sick people have gone untested — that the authorities in Hubei Province have started counting patients whose illness have been screened and identified by doctors.
The result was a sudden — and large — spike in the overall tally for the coronavirus: more than 1,300 people killed and well over 50,000 infected.
The surge in cases in Hubei, the result of a local change in how cases are counted, underscored how elusive the exact scale of the epidemic is.
The change in how cases are counted — both inside and outside the epicenter of the outbreak — reflects a two-headed problem in the global fight against the disease. On the one hand, health officials need to stay flexible in dealing with new outbreaks.
One the other hand, mistrust of the Chinese government — especially when it comes to being transparent about the threat and extent of the virus — remains pervasive.
“It’s pretty clear that there is an issue with trust about whatever the Chinese government comes out with at the moment,” said Kerry Brown, a former diplomat and director of the Lau China Institute at King’s College, London.
“That may be terribly unfair,” Mr. Brown said. But, he added, “to redefine things — even legitimately — at a moment like this is always going to be a presentational challenge, because people are going to be very sensitive, and they’re going to suspect there’s another agenda.”
The new numbers out of Hubei came only a day after China reported that new infections had hit the lowest point in a single day since Jan. 30. Experts cautioned then that it was premature to draw any conclusions from the drop.
The shifting case counts are not the only example of conflicting or spotty information. Researchers have given differing estimates on when the outbreak might peak, ranging from a date already past to several months in the future. The Chinese authorities have closely guarded the demographic details about the fatalities, creating uncertainty about who is most susceptible.
Scientists have even debated just how much of a danger asymptomatic transmission poses. Chinese health officials were among the first to raise the prospect of asymptomatic transmission, even as health experts in other countries were skeptical of early reports that the virus could be passed on by such patients. But in recent days, Chinese officials have also played down the asymptomatic transmission risk.
The uneven information comes as China’s leader, Xi Jinping, said that the containment efforts were working.
“After hard work, the epidemic situation has seen positive change, and the prevention and control work has achieved positive results,” he said during a meeting of the Communist Party leadership in Beijing on Wednesday, according to a summary of his remarks by state-run media. “This is hard-won, and all parties have contributed.”
The changes to the classification of asymptomatic coronavirus cases emerged on Jan. 29, in a set of guidance from China’s National Health Commission. The agency said that it would no longer count patients who had tested positive for the coronavirus but did not display symptoms as “confirmed cases.” Instead, those patients would be counted separately, as “positive diagnosis” patients, and would become confirmed only if they began showing symptoms.
Chinese health officials have given little public justification for the labeling change. The National Health Commission did not immediately return a request for comment.
The changes have prompted debate among some public health experts.
“Adapting definitions during an outbreak is not unusual, with increasing insights and also with prioritizing where efforts need to go,” said Dr. Marion Koopmans, the head of the viroscience department at Erasmus University Medical Center in the Netherlands.
Still, even experts who said that the effect of discounting certain cases would be small said that it would be useful to epidemiologists — and the public — to have a complete count of all cases, including asymptomatic ones.
Dr. Malik Peiris, the chair of virology at the University of Hong Kong, said in an email that mild or asymptomatic infections could form an “invisible iceberg” that made a given virus’s fatality rate much lower than it initially appeared.
Determining the true denominator of the number of people infected — whether symptomatic or not — would, he said, “be most informative to, hopefully, calm current panic in China and the world.”
In a sign of just how much the changes could further muddle public understanding of the virus, even the W.H.O., which has praised the Chinese government for its cooperation in fighting the outbreak, expressed confusion.
Dr. Chen Bingzhong, a former senior government health official in Beijing who has been vocal in calling for more transparency about public health crises, said a full tally of the number of cases would keep the authorities accountable.
He added that a positive test result should be disclosed as such, regardless of symptoms.
“Testing positive means it is the new coronavirus,” he said. “If you don’t recognize it, you are covering the truth.”
Jin Wu contributed reporting from Hong Kong, and Yiwei Wang contributed research from Beijing.
