One of the hopes of people watching China’s coronavirus outbreak was that the alarming picture of its lethality is probably exaggerated because a lot of mild cases are likely being missed.

But on Tuesday, a World Health Organization expert suggested that does not appear to be the case. Bruce Aylward, who led an international mission to China to learn about the virus and China’s response, said the specialists did not see evidence that a large number of mild cases of the novel disease called Covid-19 are evading detection.

“So I know everybody’s been out there saying, ‘Whoa, this thing is spreading everywhere and we just can’t see it, tip of the iceberg.’ But the data that we do have don’t support that,” Aylward said during a briefing for journalists at WHO’s Geneva headquarters.

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Getting a handle on how many people have actually been infected is crucial to assessing how dangerous this virus is. During the early days of an explosive outbreak with a new pathogen, it is hugely challenging to look beyond the people streaming into hospitals for care to see whether there are many more at home with a mild cold or manageable flu-like illness.

If large numbers of mild or virtually symptom-free cases are evading detection, that would suggest that estimates of the proportion of people who might end up in ICUs or might die during a Covid-19 epidemic would be lower than what has been seen to date in China.

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But if there aren’t large numbers of uncounted cases, the severity seen in China is what the rest of the world should expect as the virus moves to new locations, especially if it spreads to the degree seen in Hubei province, where the outbreak began.

“What [the data] support is that sure, there may be a few asymptomatic cases … but there’s probably not huge transmission beyond what you can actually see clinically,” Aylward said.

The claim was quickly challenged by an infectious diseases expert who serves on a committee that advises the WHO’s health emergencies program.

Gary Kobinger, director of the Infectious Disease Research Center at Laval University in Quebec, said it would be highly unusual for there not to be mild or symptom-free cases that are being missed. He pointed to the fact that outbreaks have popped up in countries far from China — including Iran and Italy — because people with mild infections were not detected and traveled to other places.

“There are mild cases that are undetected. This is why it’s spreading. Otherwise it would not be spreading because we would know where those cases are and they would be contained and that would be the end of it,” said Kobinger, who insisted that mild, undetected infections cannot be ruled out until people who haven’t been diagnosed with the illness can be tested for antibodies to the virus.

Those kinds of tests, called serology tests, are just becoming available in China, Aylward said.

“As long as we do not have good serology data, I think that it is completely speculative to say that there are no undetected cases,” Kobinger said.

Aylward pointed to an analysis from Guangdong province suggesting that, at least there, most of the infections were coming to the attention of health authorities.

When the virus started to spread in Guangdong — the province where the 2002-2003 SARS outbreak began — worried people flooded fever clinics to be tested. Of 320,000 tests performed, just under 0.5% were positive for the virus at the peak of transmission there, he said — which suggests that only 1 case out of 200 was being missed.

Transmission of the virus has subsided in Guangdong, and the number of positive tests at the fever clinics has declined; now only about 1 in 5,000 people tested at the fever clinics is positive for the virus, he said.

Aylward said that across China, about 80% of cases are mild, about 14% are severe, and about 6% become critically ill. The case fatality rate — the percentage of known infected people who die — is between 2% and 4% in Hubei province, and 0.7% in other parts of China, he said.

The lower rate outside of Hubei is likely due to the draconian social distancing measures China has put in place to try to slow spread of the virus. Other parts of China have not had the huge explosion of cases seen in Hubei, Aylward said.

A case fatality rate of between 2% to 4% would be catastrophic, if the virus spreads widely and infects a lot of people. Even a case fatality rate of 0.7% — which means 7 out of every 1,000 infected people would die — is sobering. It is seven times the fatality rate for seasonal flu, which is estimated to kill between 290,000 and 650,000 people a year globally.

An earlier version of this article included a reference to the case fatality rate of the 1918 pandemic that is contested in some quarters.