I'm wondering if there is a pool of virus in a set of cells that isn't reached by some drugs and that are not measured by blood tests. We're not talking about a lower than the limits of detection, but rather a reservoir, say in kidney cells . What's important is that if that pool is merely hiding out and escaping the drug, then longer treatment with the same drug would result in a cure. However, if that pool can replicate and the drug is not reaching those cells, then you need an additional drug.