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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 that governments will need to fund nationwide lockdowns for 10 days. Maybe longer as it takes time to learn how. So 14 days. Get everyone financed. I like the pause in debt repayments. Good idea. Support the lenders.

    Reportedly from a German investigation, the virus does not replicate or infect once the patient’s immune system is sufficiently triggered. Mostly 6-7 days from first infection.

    But lock down enforcement will be a problem. And illegal immigrants flowing into various nations like the UK Mediterranean countries and the USA undetected. How do you deal with that?

    Anyway if this is done economies will continue to work. The infection rate will be manageable.

  • Everyone needs to read a bit about local hospitals in China. While the large western-type facilities in Hong Kong, Shanghai, Beijing, and Guangzhou are okay, most are FAR different. Visualize a hospital where blood, urine, and excrement drip onto the floors…where oxygen tanks are hard to get, and even if a patient gets oxygen they are still in a very dirty bed (likely from prior patient) and in a poorly ventilated room with 50+ other ill patients, and you’ll get the picture. It is no wonder that the death rate in a Wuhan hospital is likely to be higher from COVID-19 than just about anyplace else! The US population is overreacting!

    • The current mortality rate in the US from Coronavirus is 4.08%. It’s currently 80x more deadly than the flu, and there are another 50+ people in critical condition. But yeah, overreacting… Idiot.

      Coronavirus has NO TREATMENT. It doesn’t matter what hospital you’re in. There is NO ANTI-VIRAL for it. IV Fluids do not help. In fact, they exacerbate the fluid build up in the lungs. 4% mortality rate in the US – every single case treated.

      Smallpox had a 3% mortality rate when treated – that killed 3 million a year throughout the 20th century until a vaccine was made. Spanish flu, 2.5% – killed over 50 Million in 3 years.

      But yeah, let’s sleep on this one. Keep saying that it’s not big deal and citing things like Ebola and SARS – illnesses that required direct contact – in false equivalence to a virus that spreads in air and lives on surfaces for 24+ hours.

      Mark my words, this spring won’t be the worst of it. If the communicability for this remains where it is, we’re going to be looking at 5 Million + dead globally come the fall when the temperatures and humidity drop in the northern hemisphere. Hopefully there’s some anti-viral at that point.

    • I am strongly in favor of the protective measures being taken throughout the U.S. and around the world to slow the virus. Having said that, data and the appropriate response to that data is important, especially to prevent panic. The actual fatality rate in the U.S. as of this morning (31 deaths from 1101 cases) is 2.8%. As more people are tested and are found to be positive, this rate continues to decline. The data will obviously continue to be updated week to week. However, it is important to acknowledge that over 80% of all U.S. fatalities are individuals over the age of 65 who suffered from pre-existing health problems. This is the same group of people who historically have been most susceptible to the common flu and pneumonia fatalities. Over 500,000 people die throughout the world each year due to these more common diseases and no one panics about that. My point is simply this – this is a new disease, spreading rapidly, which deserves concern and unusual measures to combat it. However, the odds of the vast majority of people in the U.S. who are not senior citizens suffering from pre-existing health issues actually dying from this disease, is going to end up being far below 1%.

    • I’m with Rick, overreacting!
      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.
      This is China not USA.

    • No…DK, it isn’t. It amazes me that the same people whining and bi*ching ( are the whiner and b*tch in this equation) about the lack of testing want to use those obviously far too small test numbers as your denominator to determine mortality rate. Many doctors are estimating that anywhere from 20 to 50 people have it that are not confirmed either because they don’t seek treatment (the CDC IS, after all, telling people if you are one of the vast majority that has mild symptoms NOT to overwhelm the hospitals) or because they aren’t given the test. So…even if we want to use your hilarious ignorant 4% number, that would make the mortality rate about 0.1%.

  • I don’t follow the logic in this paragraph. Could you explain?

    “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.”

    The last line suggest you have a formula for: % cases missed = missed cases / all cases. How do you know what the all cases or missed cases variables are from that data?

    To me, the 0.5% would imply many more missed cases, considering Guangdong province has >13 million people. Obviously, the people who want to be tested are probably higher risk… but let’s say they were 10x more likely to have ncov-19 than the general public, implying that the general public has 0.5%/10 = 0.05% undetected rate. That would imply about 13 mil * 0.05% = 6,500 cases undetected.

    Guangdong has reported about 1,300 cases as of March 7th:

    If we go with the estimate of 6,500 undetected that would imply +80% missed cases…
    Either way we are missing a ton of variables to make the claim in the last sentence. I’d appreciate it if you amend that, because the article is excellent in the data it provides but that claim could mislead many.

  • If more than 17,000 Americns died from Swine Flu a few years ago, and it had a mortality rate of only 0.007 percent (, And Coronavirus has mortality rate of 3.4%, that means it is likely that more than 8,500,000 Americans are likely to die from the Coronavirus in the near future…This is a lot of people (old or not), and not even counting the rest of the planet. Not quite a pandemic, but pretty f—ing bad.

    • I wouldn’t waste your time, Keivan. People like Bryan live in their own little world and see only what they want to see. Best to let them hide out in their bunkers and wait the end of the world.

    • All of you are wrong.
      Bryan Smith – we can not assume every person in the country will get the infection, that does not seem to happen with the flu. And your number for flu death rate is wrong, it’s supposed to be .007 not .007 PERCENT – but you are close than the other people.
      Keivian – you are technically correct, but the virus is get to a huge number of people over time. We are going to have 1,000 people dying per day in two months. And in response to the high death rate economic activity will just keep contracting.
      Trent- Ditto. This will be huge.

    • Bryan…go back to junior high school…PAT ATTENTION in math class when they bring up basics statistics this time, and then come back to us.

      Even if the mortality rate WAS 3.4% (hint: IT isn’t), it doesn’t man that 3.4% of total population will die. Luckily for your family, you are a Smith. So people might not realize that one of their family members posted such ignorant silliness.

    • Steve…when your prediction doesn’t happen, will you come back and admit how stupid you are?

      I also love how you claim that someone who is off by a FACTOR OF 100! is “closer” than people who are off in the other direction by less than a factor of 5. is FINE that you chose not to get an education. Sure, you are scared about losing food stamps. As you should be. But don’t celebrate your stupidity

    • Ikawak – I did some bad math myself, while criticizing Bryant for doing bad math. In 2010, the year he was apparently referring to, swine flu killed 17,000 people – about 1 in 18,000 in the US – but the estimated number of people who got the flu but did not die was somewhere between 40M and 80M. – so, if the actually number had been 34M,that would mean about 1 in 2,000 people who got it died. 1 in 2,000 is not .007 % it is .05%. So that is about 7 times higher than the .007% he gave.
      So, my bad math was claiming he was off by a factor of 100 when he was off by a factor of 10, roughly.
      But his math on the 3.4% leading to many millions of death would be correct – even without any assumption everyone will get it – and a lot of people are likely to get it over the next year. Even at the 1.4% death rate some people claim and only half of us getting it in the next year 300M x .014 x 1/2 = 2M, or thereabouts.

      Sure, 2M is a lot less than 8M, but it’s the worst epidemic in the US in pretty much everyone’s lifetimes. He was right, essentially.

  • My personal opinion I think that airport’s shouldn’t let anyone fly anywhere until the experts can figure this out and fix the problem at least that would prevent any more spread

    • Pretty much what I was thinking. Emergency flights that can be tracked + commerce can keep going.
      However, think about the air travel industry… not sure how that can be sustained with closure for 1 month.

  • You need to check your references. The 1918 pandemic infected 500 million people, killing between 50 and 100 million of the infected. 50/500 is 10% and 100/500 is 20%. Check sources like National Geographic, Smithsonian, and even the CDC’s own website. They SAY 2.5% mortality, but confirm the previously mentioned numbers that divide the number of deaths by the number of currently confirmed cases. And take into account there are probably many asymptomatic or mild cases that aren’t detected. There are three states in the US right now with community spread and no connection to other cases or travel.

    • They don’t know “500 million”, they don’t know “50-100 million”. No one counted up all the bodies very accurately back then, nor could they fully identify causes of deaths. We do know global population was between 1500 and 2000 million and deaths were at least 20 million, so the fatality rate for the 1919 pandemic was at least 1%. I agree 2% is likely to be an underestimate but it also depends on exactly which deaths you count and why.

      Note that for COVID, the fatality rate estimate from WHO is 1% (see Situation Reports 30 and 31), BUT that is assuming adequate medical care. The fatality rate for Wuhan proper during the peak of the outbreak is 5-10 times higher, and Iran is heading in the same direction, because hospitals are overwhelmed. When hospitals get overwhelmed, a lot of people who just need O2 support to survive long enough to beat the disease are going to die instead.

      Your statement “there probably are many asymptomatic cases that aren’t detected” is true in USA (because so far we suck at testing) but not in China. That was the whole point of the article you apparently failed to read carefully.

  • Combatting the origin of such Pandemics and empowering the knowledge and information on which damage limitation and responsible behaviour can be practiced by all ….especially at Government level!!! Diet and habit often create environments where Viruses and Disease are going to thrive – note the Cancer and heart epidemics we suffer …..To disempower individuals and markets from the sale of animal flesh killed in unclean environments and apparently a source of many health remedies yet as with Covid-19 quite the Opposite to health results – The consequences of habit and and absence of responsibility has resulted in many deaths that are ever increasing numerically and financially . To start employing more responsible patterns in our behaviour worldwide will help us all in so many ways – ask most people wether they want to live or die will answer that they want to live and create a world safe for our children and grand children and not be part of creating the opposite – We can simply Just Give up some of the more suicidal tendencies we seem so keen to empower as ways of being. Lets quit eating animals quite the way we do and definitely not with out law and order and a safety precautions system that lowers the chance of a further cause to world disorder & illness that can lead to death. LETS ALL WORK TOGETHER TO BEST HEAL THE WOUNDS OF COVID-19 & CHANGE SOME OF OUR LESS HEALTHY AND HUGELY DANGEROUS TO WORLD WAYS OF EATING MEATS THAT ARE SERIOUSLY DANGEROUS TO ALL OF US ONE WAY OR ANOTHER UNLESS RESPONSIBLE ACTIONS ARE TAKEN. CHINA SEEMS TO BE GUILTY OF A COMPLETE ABSENCE OF RESPONSIBILTY RE THE WAY IN WHICH THEY ARE SHARING THE VIRUS COVID-19 WHICH WAS ENGINEERED AND CREATED IN THEIR COUNTRY AS A DIRECT RESULT OF THEIR MISMANAGEMENT OF SLAUGHTER OF LIVESTOCK NOT FIT FOR HUMAN CONSUMPTION AND WHICH NOW EFFECTS THE WORLD AT PANDEMIC LEVEL SPREADING IT’S DISEASE SO EFFECTIVELY AND AS YET WITH NO CURE. IMAGINE A VIRUS A FIFTY TIMES MORE POWERFUL THAN COVID-19 SUDDENLY POPPING UP IN A FEW YEARS TIME THAT COULD WIPE OUT MANY MORE THAN COVID-19 VIRUS WILL. THIS IS A WARNING TO US ALL TO CHANGE SOME OF OUR WAYS OF EATING AND WAYS OF BEING TO BECOME MORE RESPONSIBLE CHOOSING FOODS AND BEHAVIOURS THAT WILL LESSEN THE LIKELY HOOD OF ANOTHER COVID-19/SARS PANDEMIC IN THE FUTURE. I PRAY WE ARE ALL ABLE TO WAKE UP A LITTLE. THE MEDICAL TEAMS COMING TOGETHER IN RESPONSE TO THE VIRUS ARE MOST TIMES AN EXAMPLE TO ALL OF US –

    • Eating animals has absolutely nothing to do with viruses. Eating healthy and exercising plays a huge part though. I never catch anything but I know less healthy environments where people are constantly sick

    • Mike Brown – I can not understand the poster you responded to, but eating animals has a hell of a lot to do with getting viruses from the animals you eat. You can’t get viruses which only effect animals from plants. Bush meat in Africa and Wildlife markets in Wuhan enabled viruses which were not previously harming humans to get to us. I am not against meat eating, but I am against eating a bunch of wild animals which does cause zoonotic diseases.

  • I would question the accuracy of undetected cases based on one Chinese province. On the other hand, Anthony Fauci and colleagues at NIH believe undetected cases are, indeed, much higher and thus mortality is much lower – along the lines of seasonal flu. I would trust Fauci’s judgement as much as anyone else at this point.

    • Fauci’s judgment may be very good, but this scientist based his opinion on reported test results, and a LOT of them, from probably the best data source we have, China. Dr. Fauci was making a guess based on experience with other societies. The difference may be that China made sure to test everyone – pretty much everywhere else, that would be voluntary, right? What if they went door to door in Wuhan and caught nearly every case?

    • all doom and gloom if you want. I guess if I had your lonely life, I would want to die as wlell. But if what you say was true (it isn’t) then how come the US mortality frate is right around 1%…and if you take away the anomaly of Washington State (with the main cluser being a nursing home…people who are EXTREMELY vulnerable) it is less than 1%. Even though the US is EXTREMELY far behind testing, with some estimates being as little as 1 out of 50 people with the virus being tested. Most estimates are AT LEAST 20 people have it for every 1 tested. Which would make the true mortality rate be less than 0.1%. Take away the anomalous cluster in Washington State and even go for the middle of that estimate of untested, and the mortality rate is 0.02%.

  • The explanation that the low rate of mortality outside of Hubei is because of draconian measures means that the mortality rate is proportional to the infection rate. Is this indeed the case?

    • Yeah, that’s an odd statement. China might, however, be focusing extra resources on isolating the elderly to limit the chance of infection in that demographic, which has by far the highest mortality rate. There’s not enough information in this article to tell, though.

    • There’s also the fact that the medical system in Hubei was likely overwhelmed- hence infection–> Medical breakdown–> Higher death rate. The isolation itself doesn’t drop death rate, but it buys the medical system time to respond.

    • The US deaths/confirmed cases is hovering just above 1%. Taking away the Washington State cases, which is not indicative of the general population since it hit the most vulnerable subsection of the popualtion, and itis less than 1%. And that is with the lack of testing causing most experts to say that at least 20 people, and as many as 50 people have hte virus for every one that is tested and confirmed.

      Using the current death to confirmed cases, and even the middle of the estimate of unconfirmed cases, and you get a mortality rate of about 0.05%, and close to 0.3% if you take away Washington.

  • The global mortality rate from the 1918–1919 pandemic is not known, but an estimated 10% to 20% of those who were infected died (case-fatality ratio). About a third of the world population was infected, and something between 1% and 5.6% of the entire global population of over 1800 million died.
    People are quoting different mortality rates. There is the total population vs total deaths and there is the case fatality rate. So the above “rivals and even exceeds that of the 1918 Spanish Flu pandemic” statement is not correct according to the data so far.

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