It's not so much that they have no effect, it's that the effect isn't enough to bring R_0 < 1, so it's going to be at best a slowing of the natural spread, whic

It's not so much that they have no effect, it's that the effect isn't enough to bring R_0 < 1, so it's going to be at best a slowing of the natural spread, which in practice, you aren't going to be able to measure or prove.