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“You may have heard several different projections about the fatality rate of COVID-19. How do different health organizations come up with these figures, and why do the numbers seem so fluid?

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Last week, the World Health Organization announced that the fatality rate of COVID-19 is higher than we thought. On March 3, they noted that, worldwide, about 3.4% of the people who had contracted the infection had died from it, instead of the 2% figure that had been making the rounds.

There have been all kinds of reactions to that, but no matter how you feel, the key thing to understand is that this number is fluid and squishy, not some unchanging property of the virus. Right now, we don't know how deadly this outbreak will be globally, because predicting epidemics is really hard. But what we all do right now can have a big impact on that final toll.

You might think pinning down how dangerous a disease is—or what epidemiologists would call its virulence—would be pretty straightforward. After all, some statistics that get at this, like Case Fatality Rate or Ratio, are deceptively simple. You just take the number of cases, divide by the number of deaths, and boom, you have the CFR—and can go about comparing it to other outbreaks.

For example, you might have seen people saying that, on average, the seasonal flu's CFR is only about 0.1%, which makes the new coronavirus 34 times as dangerous. But that comparison is misleading, as the flu's fatality rate can vary between seasons, and COVID-19's is likely to change. See, a virus's deadliness varies based on the unique circumstances of an outbreak, and we've been very wrong in the past when we tried to predict the CFR too early.

On the one hand, that 3.4 number could be an underestimate, because COVID-19 isn't instantly fatal. People who have died so far have done so two to eight weeks after their first symptoms. So, some of the people currently counted as survivors may not end up that way.

On the other hand, it might be an overestimate because most areas are only testing people who are clearly symptomatic. So, the CFR might not be taking into account a large number of symptom-less and mild cases. Because of these and other sources of error, experts say it can take months before you can accurately predict the eventual CFR for a particular outbreak.

So, it may be too soon to tell where COVID-19 will fall. Like, right now, experts disagree about how many milder cases are being overlooked. Some think there aren't a whole lot, while others think there are tons—enough that the actual CFR is already below 1%.

That seems to be the case in South Korea, for example, where health officials had diligently tested 196 thousand people as of March 9. Their CFR is hovering around 0.7%. That might go up a bit if people who are very sick right now don't pull through, but it may not ever hit that global average of 3.4%.

Which brings us to another reason CFRs can be a bit misleading: They can vary a lot geographically. For example, within China, the CFR of COVID-19 has varied from about 4% to less than 1%. Part of the reason for that is that things outside of the disease itself—like public health infrastructure, resources, and policies—can have an impact on fatality rates.

That's because deaths tend to creep upward when health systems become overburdened. If there aren't enough doctors, hospital beds, and supplies to give every person with serious symptoms the best care, then people may die who would not have otherwise. This is why you see a lot of people harping on about flu shots in places like the US.

The two diseases have similar symptoms, so treating flu cases can drain the resources needed for treating COVID-19. And non-disease factors are why it could be really helpful to slow the spread of COVID-19, even if we can't stop it. This is what everyone on Twitter is referring to with the hashtag “flatten the curve.” The idea being that, overall, the same number of people might get infected with the virus, but if the cases trickle in, they're less likely to drown facilities.

And that means more people may survive. So just by washing your hands a lot and staying home if you're sick, you can help free up resources so health care workers can focus on patients with COVID-19. I mean, that's not the only reason to do those things.

You'll also reduce the odds that you'll get the virus… which might be worse for you personally than the CFR suggests. Or, better. See, CFRs can help public health professionals figure out how to best respond to an emerging disease.

But they don't do a great job of conveying your individual risk. 4 Let's compare COVID-19 to the seasonal flu again, because, well, everyone seems to want to do that. Even if both diseases had the same CFR, they might not pose equal risks to you. Influenza tends to hit both ends of the age spectrum pretty hard.

But COVID-19 has kind of ignored younger folks so far. No one under the age of 10 died in the first 100,000 cases. Meanwhile this year's flu has killed 125 kids and been especially bad for children in general.

We don't fully understand why these diseases have such different effects on young humans. But, it may have something to do with how developing immune systems react to different kinds of viruses. Researchers have found that the immune systems of older mice react much more strongly to coronaviruses than those of younger mice.

Something similar could happen in humans. And an immune response that's too strong can predispose people to the deadlier symptoms of respiratory infections. Aside from age, things like other health conditions and past infections might influence how you react to being infected by this new coronavirus.

You can't get any of that from the overall case fatality rate. Plus, you don't have to die for a disease to wreak havoc on your life. In fact, some research is trying to quantify the severity of an outbreak by measuring other things, like the hospitalization rate, instead of just fatalities.

No matter what, even if your individual risk of a severe coronavirus infection is low, we should all take this epidemic seriously, because what we do right now matters. We have the chance to shape how this plays out. And if we do all those things you keep hearing about—like getting a flu shot, staying home when sick, and washing our hands well and often—we can keep this thing from becoming the worst case scenario.

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