Why do case fatality rate (CFR) for #COVID19 #SARSCoV2 #hCoV19 differ so much between countries? Estimates vary from ~>7% (Italy, Iran) to <1.5% (S Korea, Germany). Are they different viruses? Does the virus treat people differently?

No & no! CFR is a slippery number...

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There are many things to take into account - CFR always varies by location, method of counting, underlying practices - & perhaps most importantly, time. I won& #39;t be able to cover it all.

But lets focus on a top few.

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First, we see *no evidence* that this CFR difference is explained by & #39;different viruses& #39;. The samples we have are actually remarkably similar - max of ~30 differences out of >29,000 bases between two samples!

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Also, these samples are *very well mixed* - almost every country has samples from every type of diversity circulating!
Samples from USA (red) & UK (yellw) intermix. Many countries are like this. Tight clusters are related only to intense sampling in some place (Washington)

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So why might case fataility rate (CFR) in #COVID19 #SARSCoV2 #hCoV19 vary so much then?
Four things to remember:

1. CFR depends on testing https://abs.twimg.com/emoji/v2/... draggable="false" alt="🧪" title="Test tube" aria-label="Emoji: Test tube">https://abs.twimg.com/emoji/v2/... draggable="false" alt="👩🏻‍⚕️" title="Woman health worker (light skin tone)" aria-label="Emoji: Woman health worker (light skin tone)">
2. Death counts always lag https://abs.twimg.com/emoji/v2/... draggable="false" alt="☠️" title="Skull and crossbones" aria-label="Emoji: Skull and crossbones">https://abs.twimg.com/emoji/v2/... draggable="false" alt="⏱️" title="Stopwatch" aria-label="Emoji: Stopwatch">
3. Population is important https://abs.twimg.com/emoji/v2/... draggable="false" alt="👵🏻" title="Older woman (light skin tone)" aria-label="Emoji: Older woman (light skin tone)">https://abs.twimg.com/emoji/v2/... draggable="false" alt="👴🏻" title="Older man (light skin tone)" aria-label="Emoji: Older man (light skin tone)">
4. Hospital capacity matters https://abs.twimg.com/emoji/v2/... draggable="false" alt="🏥" title="Hospital" aria-label="Emoji: Hospital">https://abs.twimg.com/emoji/v2/... draggable="false" alt="🚑" title="Ambulance" aria-label="Emoji: Ambulance">

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1. CFR depends on testing https://abs.twimg.com/emoji/v2/... draggable="false" alt="🧪" title="Test tube" aria-label="Emoji: Test tube">https://abs.twimg.com/emoji/v2/... draggable="false" alt="👩🏻‍⚕️" title="Woman health worker (light skin tone)" aria-label="Emoji: Woman health worker (light skin tone)">

CFR is calculated as # dead https://abs.twimg.com/emoji/v2/... draggable="false" alt="➗" title="Heavy division sign" aria-label="Emoji: Heavy division sign"> # confirmed cases. If you test a lot, the bottom number gets bigger - so your % will get smaller! Germany & S Korea are doing *lots* of testing!
If only sick tested, bottom number gets smaller - % gets larger

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2. Death counts always lag https://abs.twimg.com/emoji/v2/... draggable="false" alt="☠️" title="Skull and crossbones" aria-label="Emoji: Skull and crossbones">https://abs.twimg.com/emoji/v2/... draggable="false" alt="⏱️" title="Stopwatch" aria-label="Emoji: Stopwatch">

#COVID19 doesn& #39;t kill suddenly - it takes days/weeks to die. So the # dead will always be a few weeks behind # cases. At beginning of outbreak this effect can be particularly strong as not as many people have died as will eventually

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3. Population is important https://abs.twimg.com/emoji/v2/... draggable="false" alt="👵🏻" title="Older woman (light skin tone)" aria-label="Emoji: Older woman (light skin tone)">https://abs.twimg.com/emoji/v2/... draggable="false" alt="👴🏻" title="Older man (light skin tone)" aria-label="Emoji: Older man (light skin tone)">

#COVID19 is most risky for elderly people. In countries were the population is older, or where its introduced more into elderly groups, the # deaths will be higher. This effect can be stronger at begin of epi, if elderly groups infected 1st

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4. Hospital capacity matters https://abs.twimg.com/emoji/v2/... draggable="false" alt="🏥" title="Hospital" aria-label="Emoji: Hospital">https://abs.twimg.com/emoji/v2/... draggable="false" alt="🚑" title="Ambulance" aria-label="Emoji: Ambulance">

In places where the outbreak is very bad (Spain, Italy, Iran), hospitals are struggling to cope. They don& #39;t have enough staff/beds/supplies. This means people who might have been saved otherwise, will die - increasing the CFR.

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So when you see CFRs being compared in the news, remember, these numbers are important, but there& #39;s lots to keep in mind & consider when making these comparisons! Often reasons are more complex than simple explanation.

Help do your part 2 reduce CFR: #StayHomeSaveLives!

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(This twitter thread was inspired by a question asked by @iwan1988 - thanks Iwan!)