here are a couple links on it for the Covid19. https://www.mcgilltribune.com/sci-tech/montreal-researchers-propose-a-treatment-for-covid-19-170320/
https://regenexx.com/blog/coronavirus-episode-9-can-quercetin-help-covid-19/
I don't have any research on quercetin for cluster headaches, it was just putting a few things together and then checking it out on myself and found that 1,000mg in the morning each day now that I'm in my cycle, has brought a normally killer time, to a near stand still. It can take 20 minutes or so on the oxygen concentrator to kill an attack. Sometimes it just keeps it at bay and stops it from getting worse, but it may still hang for several hours. It is a very well appreciated therapy for me. Got it beside my bed. Since taking the quercetin, the couple times I did have a problem, it went away very quickly on the O2 concentrator.