Here is a perspective from someone who has been on hydroxychloroquine (aka plaquenil) for couple of years for my Rheumatoid Arthritis. In Rheumatoid Arthritis, the immune system goes bananas and attacks various parts of your body instead of protecting your body from invaders. In my case, it attacked my heart. I now have severe heart failure and have had 3 heart surgeries with 2 new artificial heart valves. The hydroxychloroquine and other drugs are used to tamp down the immune system, so the immune system won’t attack you.
It is my understanding that the Covid-19 puts your immune system in hyperdrive, and that the inflammation and damage are actually caused by your own immune system attacking you, not directly by Covid-19, which is why these drugs are effective against Covid-19.
According to my rheumatologist, the hydroxychloroquine won’t keep me from getting Covid-19, but will tend to keep me from getting a severe case. I do hope he is right. He is in the top of his field here in Los Angeles.
That people with RA and lupus weren't getting sick was what clued Chinese Doctors into looking at HOCQ.