VRTX's strategy makes sense since telaprevir-resistant variants were associated with single nucleotide substitutions, mutants emerged very quickly, and continuing treatment after breakthrough allows selection of multiple mutation, so checking as early as 2 weeks for suboptimal responders will minimize the risk of the selection of resistant variants.
Other points to take into consideration regarding telaprevir resistance in the future: there was a difference in risk of emergent drug-resistance between subtypes (1a more frequently than 1b), and typically different mutations between those subtypes. Also the pre-existing dominant strain in a patient's quasispecies can sometimes be the V36M or R155K variant (that has a reduced response to telaprevir), and even if a variant present at low prevalence within the quasispecies it can quickly become the dominant sequence if it offers a selective advantage.
