Dose matters enormously. Hydroxychloroquine is actutely toxic to humans, so using hydroxychloroquine requires you to balance its toxicity versus its antiviral effects. My read of the evidence is that it is ineffective to harmful at the late stages of COVID19 in the dosages high enough to "do something", but taken in the very early stage of the disease (asymptomatic) it might keep the virus contained to its area of initial infection and prevent the disease from migrating to the lungs.

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How strong is the evidence for hydroxychloroquine?

by Chris_Leong 1 min read5th Apr 202013 comments

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There has been a lot of discussion of hydroxychloroquine (see the megathread on Effective Altruism Coronavirus Discussion, note you need to answer two questions to gain access). Doctors treating COVID-19 have rated hydroxychloroquine the most effective drug based on their experience. But on the other hand, results have been mixed with a recent RCT showing no effect.

At this stage how strong is the evidence for hydroxychloroquine and if it works, how effective does it appear to be as a treatment?

Disclaimer: Please seek medical advice before taking any substance, particularly those like hydroxychloroquine that have known side effects.

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It occurs to me that chloroquine is still taken widely in malarial regions as a prophylaxis, even though malaria has developed resistance to it.

So if it worked to deter COVID19, we should be seeing very few cases in, say, Nigeria, where it's a popular over-the-counter treatment even though it's no longer recommended as first-line treatment, and in, say, the Dominican Republic, where malaria isn't yet resistant and it's still the best treatment.

Dose matters enormously. Hydroxychloroquine is actutely toxic to humans, so using hydroxychloroquine requires you to balance its toxicity versus its antiviral effects. My read of the evidence is that it is ineffective to harmful at the late stages of COVID19 in the dosages high enough to "do something", but taken in the very early stage of the disease (asymptomatic) it might keep the virus contained to its area of initial infection and prevent the disease from migrating to the lungs.

A study, not peer-reviewed:

Outcomes of hydroxychloroquine usage in United States veterans hospitalized with Covid-19

HC= hydroxychloroquine,

HC+AZ = hydroxychloroquine and azithromycin

no HC = no hydroxychloroquine

RESULTS: A total of 368 patients were evaluated
(HC, n=97; . HC+AZ, n=113; . no HC, n=158).
Rates of death in the HC, HC+AZ, and no HC groups were 27.8%, 22.1%, 11.4%, respectively.
Rates of ventilation in the HC, HC+AZ, and no HC groups were 13.3%, 6.9%, 14.1%, respectively.
Compared to the no HC group, the risk of death from any cause was higher in the HC group (adjusted hazard ratio, 2.61; 95% CI, 1.10 to 6.17; P=0.03) but not in the HC+AZ group (adjusted hazard ratio, 1.14; 95% CI, 0.56 to 2.32; P=0.72).
The risk of ventilation was similar in the HC group (adjusted hazard ratio, 1.43; 95% CI, 0.53 to 3.79; P=0.48) and in the HC+AZ group (adjusted hazard ratio, 0.43; 95% CI, 0.16 to 1.12; P=0.09), compared to the no HC group.

CONCLUSIONS:
In this study, we found no evidence that use of hydroxychloroquine, either with or without azithromycin, reduced the risk of mechanical ventilation in patients hospitalized with Covid-19.
An association of increased overall mortality was identified in patients treated with hydroxychloroquine alone.
These findings highlight the importance of awaiting the results of ongoing prospective, randomized, controlled studies before widespread adoption of these drugs.
.....
However, given its increasingly widespread use, not only as therapy but also as prophylaxis for Covid-19, there is a great and immediate need to obtain insights into the clinical outcomes among patients currently treated with hydroxychloroquine, particularly because of the non-negligible toxicities associated with its use.

Seems like Algeria and Morocco improved after starting to use HCQ;

https://twitter.com/Covid19Crusher/status/1254176105730359300