In addition to its anti malarial effects, HQ has anti-inflammatory effects (mainly through reduced cytokines production) and hence is used by rheumatologist as a 2nd line drug for a variety of conditioned.
It may also have some antiviral activity (cytokines again, but multiple additional postulated Mechanisms). These effects can be observed at high concentrations in lab tests, but it is not clear these mechanisms are significant at concentrations safely achieved in clinical use. Bleach and 70% alcohol also work in lab.
https://pmj.bmj.com/content/early/20...dj-2020-137785
However the current best evidence (Recovery trial) is that it’s not helpful for COVID.
https://www.recoverytrial.net/