We don't know if it does yet. We know it causes an immune response. We don't know if that stops you from catching it, and we don't know if it stops you from being infectious. It tips the scales in that direction, and all we really need is for it to be effective enough to drop R0 low enough that coupling it with reasonable restrictions makes the virus go away, which is why it's worth pursuing, but that's a different goal to ensuring you can't catch it at all.