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The rate at which COVID-19 kills the people who get infected with it is one of the biggest unanswered questions of the global coronavirus pandemic.

Fatality rates appear to vary dramatically between countries around the world, anywhere from 0.5 to 10 per cent of cases, and experts say the true answer lies somewhere in the middle.

In early March, before the number of reported cases entered the hundreds of thousands and the death toll from the disease spiked, the World Health Organization released an estimated case fatality rate.

"Globally, about 3.4 per cent of reported COVID-19 cases have died," said WHO director-general Tedros Adhanom Ghebreyesus on March 3. "By comparison, seasonal flu generally kills far fewer than one per cent of those infected."

That number was determined by using a simple mathematical formula — the number of confirmed deaths divided by the number of reported cases.

At the time, there were 90,893 reported cases of COVID-19 around the world and 3,110 deaths.

But the number of cases is now more than five times what it was then — with more than 500,000 cases and 25,000 deaths worldwide — putting the current global case fatality rate at just over 4.5 per cent.

Why is it different in every country?

The global case fatality rate is constantly in flux for a variety of reasons, but it doesn't necessarily mean the virus is more deadly in any one region of the world.

The onset of outbreaks in different countries, the accuracy of their reporting, backlogs in testing, the age of their population and the effectiveness of containment measures taken to stop it from overwhelming health care systems all contribute to a varying rate.

Among countries that have been hit particularly hard by the pandemic, Iran has seen a case fatality rate of more than seven per cent, Spain over 7.5 per cent and Italy above 10 per cent.

A medical worker wearing a protective mask and suit treats patients suffering from COVID-19 in an intensive care unit at the Oglio Po hospital in Cremona, Italy, on March 19. (Flavio Lo Scalzo/Reuters)

Yet other countries have fared far better given the circumstances.

South Korea and the U.S. both have a case fatality rate of about 1.5 per cent while Germany currently sits at just 0.5 per cent.

Chief public health officer Theresa Tam said Thursday that six per cent of Canadians who contract COVID-19 are hospitalized, 2.5 per cent require critical care, and Canada has a case fatality rate of about one per cent.

"The fact that Canada's fatality rate is at one per cent indicates that the health care system is not overwhelmed," she said. "But these fatalities could be reduced further by preventing illness in our most vulnerable populations."

Dr. Bruce Aylward, a Canadian doctor and epidemiologist who led the World Health Organization's COVID-19 mission to China, told CBC News the case fatality rate is a snapshot of the outbreak in each country at a given time.

Aylward, who is also a senior adviser to Tedros, said the reason the WHO came out with the 3.4 per cent estimate was to stress to world leaders the serious threat the virus posed to their populations so they had time to prepare.

Dr. Bruce Aylward, who led the WHO's COVID-19 mission to China, says focusing on how low a country's fatality rate is fails to take into account how serious COVID-19 is for older age groups in a given population. (Salvatore Di Nolfi/Keystone via AP)

"What [Tedros] was worried about was that people were cherry-picking the numbers," Aylward said in a phone interview from Geneva earlier this month.

"They were looking for the lowest possible number they could find anywhere."

He said that if world leaders didn't think the disease was severe enough, and instead compared it with something like seasonal flu, they would be completely unprepared when the coronavirus landed on their shores.

"The biggest problem we have with COVID-19 is it's all about speed, it's how fast you can identify who's infected and then basically take them out of the game in terms of transmitting the disease," he said.

"If they think it's not a serious disease … they are going to be ambivalent and we run into problems."

Age of population a key factor

Aylward said focusing on how low a country's fatality rate is fails to take into account how serious COVID-19 is for older age groups in a given population.

"People take the low numbers and that's what's got us really worried, because you're making a judgment about who lives and dies and how you value different members of your society, and we don't accept that," Aylward said.

"The reality is, if this disease gets out of control in a place, then you are going to get an overwhelmed health system, and it's very, very difficult to optimize care."

More than 80 per cent of global COVID-19 infections are estimated to be mild, meaning symptoms are manageable and not life-threatening, compared with 15 per cent that are severe and five per cent that are critical and require ventilation.



In patients aged 70 to 79, that rate increases to eight per cent, and for those above 80 years old it rises to almost 15 per cent.

"The age distribution of your population will make an enormous difference to what the case fatality rate looks like for this infection," said Dr. Allison McGeer, an infectious diseases specialist at Toronto's Mount Sinai Hospital.



"In the developing world, the average age of the population is significantly lower than it is in countries like Canada. So that will also make a significant difference."

For comparison, 14 per cent of South Korea's population is over 65, while in Italy that number is more than 23 per cent — a possible contributor to the vastly different rate at which people with COVID-19 die in those two countries.



Canada has a rapidly aging population, with 17 per cent of our population over 65. That number is expected to grow to almost 23 per cent in the next decade.

Testing essential to understanding true fatality rate

Widespread testing is a major factor in determining an accurate case fatality rate, because it allows countries to capture a wider picture of the severity of cases, including those with mild or no symptoms — which will ultimately drive the rate down.

Germany has been testing about 120,000 people per week, a key factor in why it has one of the lowest case fatality rates in the world.

Amesh Adalja, an infectious disease physician and a senior scholar at the Johns Hopkins Center for Health Security in Baltimore, Md., said a lack of testing hinders our ability to know how deadly COVID-19 truly is.

"We need to at least test enough so that we can understand what the prevalence of this is in certain communities and help us get an understanding of the true case fatality ratio," he said. "Because we know that that number is skewed and it's controversial."



Adalja says it's not universally accepted by experts that 3.4 per cent of people infected with COVID-19 will die and that the more accurate number may be closer to what was identified after widespread testing in South Korea.

A visitor gets tested at a 'drive-thru' testing centre for COVID-19 in Daegu, South Korea, on March 3. Widespread testing and a younger population in South Korea may have led to a lower case fatality rate. (Kim Kyung-Hoon/Reuters)

"Maybe that's something more accurate than what we're seeing, and maybe it's even lower than that," he said. "I think that's one of the most important outstanding questions."



Adalja says it could be months before we get the true fatality rate for COVID-19, but that if we can test for milder cases and conduct serological blood tests to find antibodies in the human population, we'll have a better idea of what we're dealing with.

McGeer said we're still unclear on what the full spectrum of illness for COVID-19 is — what proportion of people carry it without knowing compared with those who get severely ill.

"We know a lot about the severe illnesses, but I don't think we know enough about the mild illnesses in terms of what their prevalence is," she said.



"People saying that 80 per cent of cases are uncomplicated and 20 per cent are more severe, I think that that number itself is also something that would be amenable to modification under more liberal testing policies."



She said that to truly know how deadly COVID-19 is around the world, we need to rapidly scale up our ability to test for potential cases — especially in those who may not display symptoms.

"We're still missing the asymptomatic proportion, which is critical and might pull down their mortality rate even more," she said.



"What we need is the testing capacity. We need to use it and leverage that as a tool of surveillance so that we can understand where this virus sits in terms of its risk to the world."

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