Researchers want to know how many people with the coronavirus don't have symptoms.Credit: Stringer/Getty

Researchers are concerned that China’s official reports on the number of coronavirus infections have not been including people who have tested positive for the virus but who have no symptoms. They fear the practice is masking the epidemic’s true scale. But public health experts say China is right to prioritize tracking sick patients who are spreading the disease.

Since the early days of the outbreak, the country’s National Health Commission has reported daily infection counts, which infectious-disease researchers outside China have been relying on to model the outbreak’s spread and severity.

Earlier this month, officials from Heilongjiang province in northeast China announced that 13 people who had tested positive for the virus with a lab test but who had no symptoms had been removed from the region’s list of confirmed cases. Officials said that they were following the commission’s guidelines for reporting infections, which state that such people should be classified as ‘positive cases’ rather than ‘confirmed cases’. Only confirmed cases are noted in the commission’s official daily reports.

The situation in Heilongjiang has put a spotlight on China’s reporting guidelines. These had already been getting attention after they were updated on 7 February to allow physicians to confirm cases using images from chest scans rather than waiting days for lab tests. The change in diagnostic criteria saw infections in Hubei, the province at the centre of the epidemic, jump by nearly 15,000 cases in a single day last week.

Reporting requirements

Wu Zunyou, the chief epidemiologist at the Chinese Center for Disease Control and Prevention in Beijing, who helps to implement the guidelines, says that they have always required that positive cases not be counted as confirmed cases. Instead, those who test positive are isolated for 14 days and monitored by health authorities. If they develop symptoms in that period, they are classified as a confirmed case.

Wu says that a positive test doesn’t necessarily mean that a person is infected with the virus. Lab tests typically detect genetic material from the virus in throat or nasal swabs, but in some people the virus might not have entered cells and started replicating, he says. But it’s not clear whether such ‘carriers’, who test positive for the virus but are not actually infected, exist. “That is one of the big scientific questions,” he says.

But several infectious-disease researchers outside China who spoke to Nature disagree. A virus usually has to replicate inside a host to reach detectable levels, says Angela Rasmussen, a virologist at Columbia University in New York. If the virus is in a person’s nose but hasn’t infected any cells, then “I’m sceptical that virus in a nasal swab from this type of exposure would be detectable,” she says.

Omitting these cases from official counts gives the impression that the virus is more severe than it really is, says Ian Mackay, a virologist at the University of Queensland in Brisbane. This could mislead other countries that are trying to prepare for the most likely effects of an epidemic of the disease, known as COVID-19, in their population, says Mackay.

Lost contacts

Not counting asymptomatic cases also hampers efforts to model the virus to understand its extent and spread, says Michael Mina, an infectious-disease immunologist and epidemiologist at the Harvard T. H. Chan School of Public Health in Boston, Massachusetts. Epidemiologists try to piece together chains of transmission, and they might include people who are infected by have no symptoms, says Mina.

He and other researchers suspect that many more people have been infected than the 74,000-odd cases that have been reported, and that most cases are probably asymptomatic and would be positive if tested.

Tarik Jašarević, spokesman for the World Health Organization in Geneva, Switzerland, said that from a public health point of view, it makes sense for China to focus on recording numbers of patients who have symptoms because those people seem to be the ones transmitting the virus to others.

Mina says that China is probably also prioritizing care for the sick, and maintaining quarantines, rather than measuring the epidemic’s dynamics. From a clinical perspective, the exclusion of asymptomatic patients from case counts is justified, because if a person has no symptoms, they don’t need medical treatment. “If I put on my medical hat for a moment, I can understand the decision not to count these individuals,” he says.