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I was analyzing some of the data surrounding COVID-19, and was looking at the vastly different mortality rates (calculated as deaths / total cases * 100).

While there are probably many factors influencing differing mortality rates (age mainly), these variables are likely negligible compared to the main factor - number of tests issued. Countries that perform the most testing have revealed the lowest mortality rates in the world (Iceland at ~0.46% and Singapore at ~0.225, compared to Belgium at ~13.95%).

I calculated the mortality rate of New York City using the same method:

11477 Deaths / 123146 Cases * 100 = 9.32%

If we take Singapore's mortality rate as an example (assuming it's more accurate given the amount of testing), we can see that New York City's is 41.42 times higher:

9.32 / 0.225 = 41.42

If we apply the same to Iceland (which has tested massive amounts of their population, and in doing so discovered that many cases are completely asymptomatic), that makes NYC's rate 20.26 times higher.

If we then multiply these factors to the number of confirmed cases in NYC (123146), that gives us between 2,494,937 and 5,100,707 cases, or around 30-60% of the city's population!

While there's a lot more variables that need to be accounted for than in my over simplified analysis, if this is even somewhat true, couldn't that imply that these places that have been hit the hardest could be not far away from reaching a preliminary herd-immunity, and seeing a massive drop-off in infections and deaths?

Note: All data used here was the most recent as of 4/16/2020.

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    One confounding factor is likely to be that they're almost certainly not using a single internationally standardised test, because there still isn't one at this time as far as I can tell, so we'd be working on comparing apples with several varieties of not-apples. – Bitter dreggs. Apr 16 at 20:04
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    @Bitterdreggs. True, not to mention the strategy used to decide who to test would be a factor as well. Once antibody testing really gets going, we'll have a better idea – robbieperry22 Apr 16 at 20:08
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    I'm not sure using Iceland's mortality rate in any math/comparison is going to lead you to any meaningful results. With total deaths at just 8 (per worldometers.info/coronavirus) adding or removing one death will skew the math dramatically. – Mr.Mindor Apr 16 at 20:24
  • NYC clearly says "We are discouraging people with mild to moderate symptoms from being tested at this time..." so certainly there are many more cases. www1.nyc.gov/site/doh/covid/covid-19-data.page – DavePhD Apr 16 at 20:25
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    Also, not a bad question, but not on-topic as presented. Is there a notable claim to analyze? Depending on how it was presented might be better fit for Cross Validated? – Mr.Mindor Apr 16 at 20:30
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Hesitant to answer this given the preliminary nature of the knowledge, but there is one paper that proposed to use the Diamond Princess (since it's a "finished" natural experiment) as a proxy to calibrate the CFR (case fatality ratio) in other populations/countries. One needs to be careful to adjust for the different age-structure of populations/samples when doing this, as well as (properly) infer the delays to death reports.

TLDR story, using this method they adjusted down the CRF in China to less than a third of what was officially reported (up to then). The Nature news coverage of that paper:

Another team used data from the ship to estimate that the proportion of deaths among confirmed cases in China, the case fatality rate (CFR), was around 1.1% — much lower than the 3.8% estimated by the World Health Organization (WHO).

The WHO simply divided China’s total number of deaths by the total number of confirmed infections, says Timothy Russell, a mathematical epidemiologist at the London School of Hygiene and Tropical Medicine. That method does not take into account that only a fraction of infected people are actually tested, and so it makes the disease seem more deadly than it is, he says.

By contrast, Russell and his colleagues used data from the ship — where almost everyone was tested, and all seven deaths recorded — and combined it with more than 72,000 confirmed cases in China, making their CFR estimate more robust. [...]

The group also estimates that the infection fatality rate (IFR) in China — the proportion of all infections, including asymptomatic ones, that result in death — is even lower, at roughly 0.5%. The IFR is especially tricky to calculate in the population, because some deaths go undetected if the person didn’t show symptoms or get tested.

See also a longer, more technical explanation/quote I gave from the actual paper on med.SE.

Also, beware this is not the end-all to modelling such matters since e.g. whether the healthcare system gets overwhelmed or not may matter a fair bit (to CRF) and this factor may differ between countries, but that probably won't get captured well in simple adjustments by population-age structure.

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The precise question being asked here is not that clear to me, but here is a brief and accessible article from The Lancet a few weeks ago which outlined the challenges of calculating COVID-19s general CFR (Case Fatality Rate). It concludes:

Although highly transmissible, the CFR of COVID-19 appears to be lower than that of SARS (9.5%) and Middle East respiratory syndrome (34.4%), but higher than that of influenza (0.1%).

In time we'll have a better idea what the real average CFR is, but based on the case of the Diamond Princess, these authors suggest it is something approaching 1% or a little less.

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