It isn't any surprise that Ivermectin doesn't work for severe Covid. Severe Covid is a runaway inflammatory response unrelated to viral load, we know that now. In fact, most people dying of Covid don't have the replicating infection anymore, their immune system is actually killing them several weeks later. Any vaccine will work for Covid, e.g. MMR, because what you need to keep yourself out of the hospital is concept called Innate Immune tolerance.