The race to track and slow the coronavirus pandemic has been very much a numbers game.

While we watch the number of cases — and deaths — climb, experts also are closely eyeing what they call the case fatality rate (CFR). This tells the proportion of people who have been confirmed as having Covid-19 and ultimately die from the illness, and it’s a number that has been varying widely from country to country.

Some countries, such as Germany, have a fatality rate of approximately 3 percent of confirmed cases, whereas Italy’s rate has climbed above 13 percent. Even within the US, large differences have emerged: As of April 20, Michigan had reported a CFR of 7.6 percent, compared to Oregon’s 3.8 percent.

Since scientists are pretty sure the virus isn’t mutating very quickly, the pathogen itself shouldn’t be more deadly in one place than another. So why the variation?

Figuring out what is driving these disparate numbers reveals differences not just in population demographics but also in health care capacity and government response. Understanding CFRs might also help us slow deaths around the world — especially for the most vulnerable countries and individuals.

Why might fatality rates be so radically different?

Before diving into the details from different countries, it’s worth looking at some of the broad reasons why death rates can look so starkly different from place to place.

First, we know that there are big differences in the risk the virus poses to different age groups. For this coronavirus, SARS-CoV-2, older individuals are far more likely to become critically ill or die from the disease. In a paper in The Lancet Infectious Diseases, researchers concluded, when looking at data from China and elsewhere, that people between the ages of 40 and 49 have an estimated CFR of about 0.4 percent; for those 80 and older, it’s 13.4 percent. This gulf of survivability is already playing out in some countries with older populations, such as Italy.

Additionally, Covid-19 has been demonstrably deadlier for those with existing health conditions, including lung disease (often caused by smoking), cardiovascular disease, severe obesity, diabetes, kidney failure, and liver disease. So countries — or regions — with less healthy populations might also be seeing big differences in the rates at which people are dying from the illness.

Beyond the varying impacts of the illness itself, there are lots of variables in how numbers are being gathered and reported. Perhaps the biggest factor here is testing. When experts calculate a basic fatality rate, it can be as simple as dividing the number of deaths by the number of confirmed cases (although — and we’ll get to this later — it really shouldn’t be).

Since the international spread of the novel coronavirus, countries have varied widely in their ability and willingness to roll out testing. So that means the denominator (the number of cases) can be closer or further from an accurate count of how many people actually have the virus. The larger the percentage of a population that has been tested, the more complete picture we will get of the virus’s actual fatality rate there.

The other issue with the poor testing rates is sampling bias. Tests that are available are usually saved for the sickest and riskiest cases. This pushes the fatality rate higher than it actually is because the testing is more likely to omit mild or asymptomatic cases and instead overrepresents those who are more likely to die. So, as testing becomes more widespread in various countries, their fatality rates will drop.

That is no reason for optimism, as the authors of the study in The Lancet note. The researchers offer an overall CFR for Covid-19 at 1.38 percent, which reflects their estimates for lack of testing and other factors, including potential censorship. This number, they noted, is still “substantially higher than for recent influenza pandemics (e.g. H1N1 influenza in 2009)” — “swine flu” — which had a case fatality rate of 0.1 percent. Their estimated CFR, “combined with likely infection attack rates (around 50-80 percent), show[s] that even the most advanced health-care systems are likely to become overwhelmed.” It is clear that this is far worse than the seasonal flu.

Another way to look at death rates — in the absence of widespread testing — is to compare the number of Covid-19 deaths to a country’s total population, which is what researchers have done here.

In another effort to make up for incomplete testing (and possibly incomplete reporting), researchers are attempting to estimate what percentage of actual cases have been reported (as of a couple weeks ago) for each country.

The other factors likely impacting the vastly different fatality rates include a country’s resources (particularly its health care capacity), its organization (such as how easily it can institute effective, widespread public health measures), and how forthcoming it is with data.

We will likely see other factors emerge as the pandemic rages on and more data comes in. But here’s what we know so far about some of the key countries and their fatality rates for Covid-19 based on the number of confirmed cases.

For each country, we are using April 20 data from the University of Oxford’s Center for Evidence-Based Medicine, which has been updating its statistics on dozens of countries’ fatality rates daily. You can find their full list of countries, which includes data on how confident they are in their estimates, here.

China

Estimated CFR: 5.6 percent

Early on in any pandemic of a potentially fatal infectious disease, the death rate is going to be high for numerous reasons, including: health workers are not looking for the new disease and will miss cases early in their progression; there aren’t established treatment protocols for the illness; and people don’t yet know how it is spread (and, thus, how best to contain it).

In the first few dozen patients in China, all of whom were already hospitalized, the CFR was 15 percent, a paper in The Lancet noted. Although the rate has pushed down over subsequent weeks, as the origin of the outbreak, China’s relatively high overall rate is not a surprise.

Even though the first reported Covid-19 death occurred there January 11, experts still do not agree on what the country’s actual CFR should be. One group of researchers reported in Nature Medicine that, as of the start of March, the fatality figure there should be 1.4 percent of symptomatic cases. Another team, writing in The Lancet, estimated it should be 5.6 percent.

These sizable differences come from balancing considerations of estimating the capacity of health care systems to identify cases and deaths, determining the ability to approximate actual rates of infection (including those not tested), and accounting for the time delay between onset of illness and death (tabulating current deaths with current infections might be misleading because of the two- to eight-week lag from early symptoms to death), among other factors. Clearly, finding the CFR is not as easy as an exercise in long division.

China’s governmental response, facilitated by being relatively centralized, likely played a large role in eventually slowing the outbreak — and lowering the fatality rate — there. As more people were able to be tested and the hotspot of Wuhan was put on lockdown, fewer cases emerged, and those that did could be detected earlier. New evidence, though, also reveals that China has underreported the number of cases and deaths from Covid-19.

Italy

Estimated CFR: 13.22 percent

Italy currently has the highest fatality rate for Covid-19 of any country with a major outbreak. Why? One factor in this, experts have suggested, is the relatively older population of the country. The novel coronavirus is considerably more fatal with each passing decade of life. As a March 23 viewpoint in JAMA noted, those 60 and older in Italy have a 3.5 percent chance of dying if they get Covid-19; those 80 and up face a 20.2 percent chance.

And Italy has one of the oldest populations in the world. According to the World Bank, nearly a quarter of the country’s population is 65 or older, a higher percentage than any other country except for Japan. (There are also numerous other factors, including a cultural tradition of physical closeness, that could be contributing to Italy’s overwhelming number of cases and high death rate, as this piece from The Conversation points out.)

But that doesn’t entirely explain the country’s high number. (As we’ll see below, Germany, which also has a fairly old population, has a relatively low Covid-19 fatality rate.) Some of Italy’s dire death rate might be due to how slow it was to do widespread testing.

Italy has now performed a lot of tests — about 22 per 1,000 people as of April 20 (compared to about 11 per 1,000 in the US). But it was, compared to countries like Germany and South Korea, relatively slow in the early stages of its outbreak to make large numbers of tests available, allowing the virus to spread undetected for crucial periods of days and weeks while people went about their daily lives, unaware that a crisis was looming.

Germany

Estimated CFR: 3.19 percent

Experts are not sure why Germany has managed to maintain a relatively low fatality rate for Covid-19.

As HJ Mai reported in Vox, part of it does likely have to do with their aggressive early testing:

The [country’s Robert Koch Institute] early on recommended broad testing to detect cases as soon as possible and to slow the outbreak. “This is probably why we started to see cases very early, also mild ones, which in other circumstances might have been missed,” RKI’s [deputy spokeswoman Marieke] Degen said. “If you start seeing deaths, it indicates that the virus has already been active in the community for some time.”

Another benefit of the widespread early testing is that they were able to locate a larger number of cases — adding to the denominator — many of which were milder and less likely to lead to a death. As Mai wrote, “The majority of cases in Germany have been detected in people between the ages of 35 and 59. That most coronavirus cases in Germany are being detected in an age demographic that is not considered part of the high-risk population could be a further contributing factor” to the low fatality rate.

But there is another question of timing here, and that is when a country’s outbreak started. Germany’s outbreak began later than Italy’s, with Italy topping 1,000 cases by March 1, when Germany still was reporting just slightly over 100. And with the current estimates suggesting Covid-19 deaths occurring up to eight weeks after the onset of symptoms, as Mai notes, “That means there’s a chance Germany and other nations in a similar stage of the outbreak could soon see a spike in deaths.” Indeed, their fatality rate has been climbing slowly over the past weeks, rising from about 1 percent at the beginning of April.

Related Why France has 4 times as many coronavirus deaths as Germany

South Korea

Estimated CFR: 2.21 percent

Widespread and early testing, however, is not all that it takes to keep infection and death rates low. The results of those tests also need to be swiftly acted upon to have the most benefit.

South Korea has been largely lauded for its rapid, robust, and coordinated response to its outbreak. In addition to having tested about 3 people per thousand by March 5 (which was days before the US even started testing anyone in the general population), they put the results of those tests to use right away, an article in Science explained. Positive cases were isolated. Additionally, that person’s contacts were traced, and they were themselves considered potential cases and asked to quarantine at home, as NPR reported.

New daily confirmed cases there spiked to their highest level March 1 — at 730 per day — and then dropped off, reaching an average of just 67 new cases per day by April 1. (By comparison, the US had 777 reported new cases per day on March 15 — and, as of April 15, had more than 26,500 new cases per day.)

To facilitate this massive undertaking, the country used GPS data from people’s phones (because your phone is better at remembering where you were and who you were near than you are). The approach raises privacy concerns, as Science pointed out, but it is also credited with helping to sharply curtail the country’s outbreak.

Japan

Estimated CFR: 2.19 percent

Japan, a country with more than 35.8 million people 65 or older, has many global health experts worried. The government has lagged on testing as well as on instituting strict social distancing guidelines. As of late March, many shopping malls in Tokyo were still open and busy. And as of April 15, the country had performed just about 1 test per thousand people (which pales in comparison to even the paltry efforts of the US’s 10 tests per thousand people by the same date).

As Eric Margolis reported for Vox, these facts do not bode well:

“Thus far, Japan has managed to escape exponential growth, but the worst may be yet to come. ‘This may be the tip of the iceberg,’ said John Ioannidis, professor of disease prevention at the Stanford School of Medicine. ‘If you don’t test, you find no cases and even no deaths.’”

What has people puzzled is the country’s relatively constant numbers of reported Covid-19-related deaths, despite rising case numbers. (There have been some questions as to whether Japan is thoroughly testing those with pneumonia, a common outcome and cause of death for severe Covid-19 cases.) Some attribute this to effective contact tracing that was done of cases early on in the outbreak or the country’s strong health care and elder care systems, Margolis noted.

Time — and increased testing — will tell whether Japan has efficiently controlled the virus and deaths as South Korea has managed to do. Or if it was a case of delayed detection and containment leading to a much larger disease burden.

United States

Estimated CFR: 5.31 percent

Where does the US stack up in all of this? Given that testing rates have been exceedingly low especially early on, as in Italy, undetected cases continued to spread the virus. As Dylan Scott and Rani Molla of Vox have written, case numbers in the US were more in line with Italy and Iran in mid-March, and “far outpaced places like Hong Kong and Singapore, where the governments mobilized more quickly.” The US is now on an even sharper trajectory of new cases than Italy or Iran. And that means a spike in deaths may well be coming next.

That is one of the other factors that can lead to a higher CFR: a sudden jump in the number of severe cases. When that happens, it “adds to the strain on the healthcare system and can overwhelm its medical resources,” wrote a team of researchers in The Lancet. That can result in not being able to save as many lives. (Hence the call to “flatten the curve.”) The US might be looking at this scenario in many places. New York City is already running out of hospital beds, with the projected peak outbreak there predicted to still be weeks away. And other cities and towns might face similar fates.

Risk map of #COVID19 in America? Here's mine: Any urban center with areas of high population density, esp those with #poverty/crowding where practicing social distancing can be challenging. So far it's been a good predictive map: NYC, Chicago, Detroit, NOLA, ATL, LA, SF, Seattle pic.twitter.com/WuQGMb7z1u — Prof Peter Hotez MD PhD (@PeterHotez) March 26, 2020

Within the US, too, the rates of mortality are varying widely. As of April 20, Michigan had a CFR of 7.6 percent — compared to Oregon’s 3.8 percent. The reasons for these emerging regional disparities are yet to be determined, but they will likely end up including the same factors that lead to countries’ different rates, such as the area’s healthcare resources and containment policies as well as the overall age and health of the population. (Current counts for states’ case and fatality numbers can be found here.)

What experts are looking out for next

It will be some time before testing catches up and we have firmer numbers for countries’ actual CFRs. Even when that happens, there will likely remain sharp contrasts among nations. As researchers noted in a March 20 Science report, “The patchwork reflects different phases of the epidemic, as well as differences in resources, cultures, governments and laws.”

As we learn more about the fatality rates for different populations of people, though, it will hopefully help governments make the best decisions about public health policies — as well as the best allocation of resources to protect and treat the most vulnerable.

A country’s response, so far, appears to have among the largest impact on the severity of an outbreak and the subsequent number of deaths, as demonstrated by South Korea’s apparent success (which was also achieved without broadly shuttering daily life). And as Vox’s Alex Ward writes: “Saving lives is ... about how quickly, thoroughly, and effectively the government responds to the brewing crisis. Any delay, it seems, is very costly.”

One of the big questions that remains to be answered is how the overlapping factors of a country’s age demographics and its health care resources impact the fatality rate. Going off of median age or the proportion of the population 65 or older may or may not end up being the next strongest predictor.

Countries with older populations tend, on the whole, to be wealthier and healthier, with better health and social infrastructures to begin with (for example, the notoriously rich country of Monaco boasts the highest media age: 55). Countries with younger populations, on the other hand, are often places where health care and other resources are scarcer and people are already not as likely to live to late adulthood (see Niger as the country with the youngest median age, at 14; and consider that in Uganda, only 2 percent of the population is over 65).

So, as the Covid-19 pandemic continues its global spread, experts will be closely watching not just countries with a large proportion of octogenarians but also those with higher poverty rates and already-stressed systems.

These countries might also be more likely to lag behind in test availability, logistics, and reporting, so it is possible that accurate data will be delayed or perhaps never be collected to know the true numbers of those infected or who perish from the illness. But even in the US, as the Atlantic reported, we might never know the true number of deaths this coronavirus causes. Because how we determine the ultimate cause of someone’s death is often not clear cut.

Will we ever know the pure case fatality rate of SARS-CoV-2? Some scientists are looking to the natural experiment that took place aboard the Diamond Princess cruise ship, which was quarantined for more than a month. During that time, about 19 percent of the ship’s passengers and crew fell ill and were confirmed to have Covid-19. Of those 705 people with the virus, seven died, providing a CFR in a well-tested group of people of 0.99 percent. The cruise ship demographic did skew older, so, as researchers pointed out in The Lancet, “the CFR in a healthy, younger population could be lower.”

Whatever they end up being, the numbers from this pandemic will doubtlessly be studied for decades to come. As in a laboratory experiment where researchers set up different versions of the same test, changing this variable or that, the world’s countries are now part of this global test case.

With hindsight and thorough analysis, we will eventually be able to more carefully plot out steps and missteps and what combinations of factors from these experiments came to mean for different populations. And hopefully, with that knowledge, we will be able to better head off the next pandemic threat, which could be even more virulent.