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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.

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.


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.

Coronavirus Coverage: Read the rest of STAT’s up-to-the-minute reporting on the coronavirus outbreak.

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.

  • I think it is kind of important to point out, unless we get a vaccine, or become, and remain, very careful to avoid transmitting the disease, the mortality rate for those infected, and the total mortality rate in the US population will be about the same in a year or so. I mean, it could be off by a factor of 2 or 3, which is a lot, but 100 Million infected people, (over the next year) with a 3% mortality rate, will mean 3 million people die. If EVERYONE gets sick, then of course we have no hospital beds or ventilators, oxygen bottles, etc for most of them, and we get maybe 10 million dying.

    It is interesting to look at Italy and see how fast it spread – they are seeing a decline in the number of new cases but still getting hundreds of deaths per day.

  • WOW..some absolutely horrible mis-use of statistics. So…they tested 320,000 people in an area where it was just STARTING to spread, found out that 0.5% had it, and that means that everywhere, only 1 in 200 cases are not being confirmed?

    Nope. That is not how it works. And I am pretty sure you know that. You are just hoping your readers are too stupid to think for themselves. Sadly, you may be right.

    • I agree. If the virus was really that deadly, and as transmissible as it is supposed to be, then why aren’t there already millions of dead people? In any case, it should be patently obvious that the data is very biased towards severe cases — (rational) people who have no symptoms at all, or feel like they have a weird cold (like myself), are not going to bother trying to get tested when they know that there is a shortage of tests, and that those tests which are available are being rationed for those with severe respiratory symptoms. Unless Bruce Aylward can convincingly explain why the available data is representative, I find his proposition simply untenable. All he does is assert that the data is representative, without trying to justify that claim. There simply are not any random samples of thousands of people to work with. (And frankly data that’s been almost certainly at least partially tampered with by the Communist party is inherently suspect to begin with, even if would have otherwise been representative.)

    • To the person who agrees with the OP and says where are the millions of dead people – I do not think you understand what “exponential” means – when they say the virus will spread “exponentially” – meaning each person who gets it spreads it to maybe 3 others – and it take 2 weeks to get sick and another week to die. On top of that, “only” 20% get very sick and “only” about 3% die – so, 3 weeks back we had about 1,000 people who would eventually get sick enough to be tested, and about 4,000 we still do not know about, meaning we actually had 5,000 cases. The 4,000 cases we did not notice have infected 3 times, two weeks back, and they infected 3 times one week back, and now this week another 3 times. – so 27 times the number 3 weeks back means we might have 100,000 people infected already. Most of them are going to infect 3 people also, to give us 300,000 next week. Obviously I am claiming a time line which may be wrong, but the point is, huge numbers of infected people are out infecting many more people. The reason you have not seen millions of deaths yet is we are at the very beginning – we should not assume the Chinese government numbers for total deaths is true – they have very strong motives to lie – that is, to lie and claim it is not as bad as it actually is.

  • The following quote from this article seems like a classic statistical fallacy.

    Event A: person has coronavirus
    Event B: person gets tested

    P(A|B) is 1/200
    P(B|A) depends on the base rates of testing and disease occurrence.
    Your article claims 1-P(B|A) is 1/200 as if this follows logically with no further information. WTF? OMG. CoV.

    “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.”

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