Assuming, by their name (anti-sense) that these drugs take advantage of the
RNA-interference pathway, the molecules used, short interfering RNA (siRNA) don't need to have a 100% specific match to a messenger RNA (mRNA) in order to silence it. When the match is 100%, then the viral mRNA is cleaved (by means of a complex called RISC), and there is no chance of it translating into protein, whereas non-perfect matches don't result in RNA cleavage, but at the same time still block the enzymes that perform the translation (because the siRNA remain stuck to the viral mRNA). Therefore, small mutations in the virus might not have an effect in the drug's efficiency. It's a win-win situation for us.