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  • Your boss, the head of the WHO, put out a mortality rate number yesterday

  • that had us all gasping frankly.

  • 3.4% mortality.

  • I would say that number is way under 1%.

  • Unlike Italy where the mortality rate is around 10%, the rate in the US

  • is closer to 1.5%.

  • In China, there's different numbers.

  • So why are Covid-19 fatality rates so varied?

  • Well, firstly, there's not just one rate.

  • There's one that helps you understand an outbreak.

  • And then there's two more, which will tell you the risk of dying.

  • Let's take a look at how they work.

  • During the outbreak of a new disease, you likely only know that some people have

  • been infected and some of those people have sadly died.

  • If 10 people have been infected and one infection has ended in fatality,

  • is tempting to conclude that this illness has a fatality rate of 1 in 10 or 10%.

  • But that's not accurate.

  • The problem is that there's a time lag between when someone gets infected and

  • when they either recover or succumb to the illness.

  • To get a more accurate number, you should only count those cases

  • that have resolved.

  • In this example, five infections have resolved so by excluding unresolved cases,

  • as we should, our fatality rate has risen to 20%.

  • This is a useful number.

  • It's the case fatality rate.

  • It's the number that you've probably been hearing the most.

  • This number helps to guide our response.

  • We can separate big populations into smaller groups

  • and discover which are most vulnerable.

  • And we can also see how effective different treatments are.

  • Definitely helpful things to know.

  • So is this number the risk of dying too?

  • No.

  • Remember, there are two other fatality rates for that,

  • But we can't get to them just yet.

  • Why?

  • Well especially early on in an outbreak we're still missing lots of information.

  • We definitely haven't counted all infected people.

  • We're missing those that have not been tested.

  • Perhaps, because of a shortage of tests.

  • We're missing those that didn't seek medical attention

  • and we're missing asymptomatic cases.

  • If we found lots of these cases, it would bring our fatality rate down.

  • But early on, we just can't be sure about how big these groups are.

  • Counting fatalities seems simpler.

  • But it's not.

  • If a person with another serious condition gets infected and then dies,

  • is that a fatality from the infection or from the condition?

  • Or both?

  • Fatalities can be undercounted too.

  • In some places, Covid-19 fatality counts have initially only included those

  • within the Health System,

  • with delays in counting fatalities in people's homes and care homes.

  • So when the count eventually catches up the fatality rate will rise.

  • So over time, as we fill in the gaps in our knowledge,

  • our case fatality rate will get closer to the number we probably want to know:

  • the risk of dying.

  • but our case fatality rate will never get us completely there.

  • Why?

  • Because by definition we're only counting known cases.

  • We could be missing loads of hidden infections.

  • These guys.

  • To try to fill in these gaps and tell us our risk of dying we can turn to our last two rates.

  • There's the Infection Fatality Rate.

  • It tells us the risk of dying if you get infected.

  • This is achieved by randomly testing a group of people.

  • This then picks up every case,

  • giving us a truer representation of the number of infections.

  • And subsequently, what share ended with a fatality.

  • Then there's the Crude Mortality Rate.

  • That tells us the risk of dying relative to the whole population.

  • Or put another way, what portion of the population has died from this disease.

  • But because we don't know how many people will end up being infected.

  • We won't know this figure until the disease has run its course.

  • So, we have three types of fatality rate:

  • two that give us a risk of dying for those infected and whole populations.

  • And a third that, though dealing with partial information, is incredibly useful

  • for understanding of disease and understanding which treatments are most effective.

  • Knowing what these numbers are for and what information they're based on

  • means that we can use them more effectively.

Your boss, the head of the WHO, put out a mortality rate number yesterday

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B1 US fatality fatality rate infected dying risk mortality

Why coronavirus death rates are so different

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    Amy.Lin posted on 2021/01/04
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