Lorenzo, it all has to do with how your body responds to the medication. Above my head somewhat but T cells play a big part in response. I am a geno 1a and vl starting out was 1,300,000. Did not get a 4 wk PCR but had one done at 6 wks which came back around 100,000. 12 wk had not cleared as well, reading was 793. If I am not UND by 24 wks which will beSept 5th, I will cease treatment because my chances of reaching SVR are not good. Viral load drops most quickly during the first 12 weeks with geno 1's and as Kristina mentioned it is harder for us to clear than other geno's. Not to say significant drop in viral load can't occur after 12 weeks but they want at least a 2 lg drop within that time period. Extending treatment to 72 wks if not UND by 12 wks gives geno 1's a greater chance of SVR.
I know you're a g1a, so hopefully some others will chime in with info for that genotype, which is a lot different from g2 and g3 (I'm a 3). The hope for g2/3 is RVR and UND by week 4 for a 24 weeks tx, but G1 takes longer, and in the first 4,8,12 weeks the log drop of the VL is more important and UND is not expected early. Some other G1's will be able to give you more details. Cheers, Kristina