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In the great Ivermectin debate, one frequently mentioned argument on the pro-side is a comparison among African countries. They argue that a set of countries in Africa having significantly lower cases and mortality are the countries with endemic parasites using Ivermectin.

Is there a correlation between Ivermectin usage in countries and lower COVID-19 prevalence?

Source: https://www.medrxiv.org/content/10.1101/2021.03.26.21254377v1

Conclusions The morbidity and mortality in the onchocerciasis endemic countries are lesser than those in the non-endemic ones. [...]

Another source: http://www.scielo.org.co/scielo.php?script=sci_arttext&pid=S1657-95342020000400201

Conclusions:

The incidence in mortality rates and number of cases is significantly lower among the APOC countries compared to non-APOC countries. [...]

It seems to be a popular claim going against common wisdom, so I am interested if it is true.

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    I've seen someone debunking this one recently, but I can't find it again. As far as I remember one huge problem is that the COVID data in Africa is unreliable, not all countries are testing and deaths might simply not be attributed to COVID.
    – Mad Scientist
    Sep 14 at 20:08
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    @MadScientist "Africans can't count" doesn't sound like a strong argument. Are there countries, which use ivermectin on a massive scale for parasites, which have high cases? That might be worth looking into. Sep 14 at 22:35
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    @MadScientist it's not "Africans can't count" it's "We don't have enough information and while it's possible, the massive number of both known and unknown factors make this study far from 'proof'".
    – DenisS
    Sep 14 at 23:13
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    There are three questions mixed in here, and it would be well to separate them: 1) Is there a correlation between countries with Onchocerciasis and lower COVID-19 cases? 2) Is there a correlation between countries with high Ivermectin usage and lower COVID-19 cases? 3) Does Ivermectin prevent COVID-19? The first two are merely intriguing observations, if true. The last one is the only one that matters and should be answered with a far wider range of evidence. Which do you want addressed?
    – Oddthinking
    Sep 15 at 2:38
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    Here's a more specific claim w/graphic: twitter.com/GidMK/status/1437559870480269314 (I'm providing it in context of someone pointing out why it's nonsense because I don't want to spread it without that context)
    – CJR
    Sep 15 at 17:34
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The first really important question is what exactly it means that those countries use ivermectin or are part of APOC. Both papers seem to assume that this status is binary, a country is either in the ivermectin/APOC group or not. But what exactly does this mean for the use of ivermectin in both groups?

APOC is the WHO African Programme for Onchocerciasis Control. I'm using the final report for this program to try and understand what exactly defines this group of countries the two papers are examining.

One important first aspect is that APOC has been finished in 2015. So this does not describe any current efforts, but a past program. Another important question is how much ivermectin was actually distributed. If ivermectin were the cause for the discrepancy a very large percentage of the population would have to be taking ivermectin. The first paper states an almost 10 times higher rate of death due to COVID in the non-ivermectin group, such a huge effect would not be possible if only a very small percentage of the population take ivermectin.

One page 24 of the APOC final report you can see how many tablets of ivermectin were distributed. At the end in 2013 this was 225 million per year for all APOC countries combined. Using the data in the first paper, the total population of all APOC countries is 972 million people. Acoording to this PLOS paper a treatment of ivermectin is 2.8 doses on average, and the estimated number of treatments was 92.5 million.

So only about 10% of the population in these countries were actually treated with ivermectin under this program. This does not seem enough to explain the dramatic difference the two papers state. And keep in mind this data is outdated, it might be even lower today.

Another huge potential confounding factor is the accuracy of the data for these countries. The number of tests varies dramatically between countries, the data is simply not directly comparable:

Presently, no country or continent knows the total number of people infected with COVID-19, despite the confirmed cases. The counts of confirmed cases largely depend on how many people have been tested in each region. To properly monitor the spread of the virus, countries need widespread testing. The African region has about 1.2 billion population, and about 2.4 million people have been tested for COVID-19. However, the continent has a testing target of 8000 tests per million population [3]. South Africa has conducted 54, 224 tests per million population, with Egypt conducting 1, 317 tests per million population and Nigeria conducting 1, 504 tests per million population [4]. Among European countries, the UK has conducted 266, 500 tests per million population.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7485446/

If a country has performed 40 times as many COVID tests per population than the country you're comparing it to, you won't get any useful data out of this comparison.

Drugs also don't stay in your body forever, so the number of people that had taken ivermectin just at the time they are also getting infected with COVID is much smaller.

The plasma half-life of ivermectin in man is approximately 18 hours following oral administration.

https://www.merck.com/product/usa/pi_circulars/s/stromectol/stromectol_pi.pdf

The half-life of ivermectin is 18 hours, so after a few days there isn't much left of it.

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