A Portugese study claiming HCQ prevents COVID

A recent preprint, "Chronic treatment with hydroxychloroquine and SARS-CoV-2 infection" by Ferreira et al. reports on a study of data from Portugal. In Portugal, there is anonymized data available on who gets prescriptions, who is tested for COVID, and who tested positive. Ferreira et al. analyzed this data to see if the people taking hydroxychloroquine (HCQ) tested positive for COVID at a higher or lower rate than the general population. They found that the relative risk of testing positive for COVID among patients taking HCQ was about half that those that weren't.  Based on these findings, the authors suggest using HCQ as a preventative measure for preventing infections of COVID (with caveats about monitoring usage, and praising it as an inexpensive drug).

Based on the issue that we need to be careful who we measure that I wrote about before, I completely disagree with this assessment. People who were prescribed HCQ in Portugal are not representative of the general population. For starters, they have rheumatoid arthritis, lupus, or another autoimmune disease; those taking it for malaria had too small a dose to be included in the study. My assumption is that people with arthritis severe enough to require medication are less likely to be going out and about as much, and will mix less with other people. These less active people are less likely to be exposed to COVID, leading to the reduced infection rate. Am I right? I don't know. But my story is consistent with the data, and an assumption that HCQ does nothing for COVID. Hence the study doesn't show HCQ helps, and certainly doesn't justify pursuing any changes of policies or medical interventions.