It works in all stages, prevention (of virus replication, not infection) through infection and even post disease if people have lingering symptoms. Here is a presentation that describes the function of ivermectin.

https://www.youtube.com/watch?v=GZoBAuR4ajs

Wife and I are taking it for prevention. Just talked to my doctor yesterday (about another topic) and the topic of ivermectin came up. He said he has no problem prescribing it to his patients if they request as he has used the drug for other purposes and it is a safe drug. 

For what it is worth. Not medical advice from me, just what we have been doing.