In figure 2a the actual viral load measurements (red dots) and the fitted viral decline curve using Eq. 1 (black line) is presented of one of the patients (No 1-I) who receive 10 MU INTERFERON daily for 28 days. The actual decline in viral load in this patient is best described by the original bi-phasic model. In figure 2b,c,d the actual viral load (red dots) and the fitted viral decline using the modified model allowing for a change in INTERFERON efficacy as described in Eq.5 is presented (black line). The actual viral decline could only be correctly fitted by using our modified model. The red lines represent the calculated bi-phasic decline curve for a continues 10 MU INTERFERON regimen in the patients of group 2 by fitting only the observed viral load during the treatment period of 10 MU daily in the original model. The green line represents the calculated viral decline curve as if a continues treatment of 3 MU INTERFERON daily was given from the start of treatment (only the viral load during 3 MU INTERFERON are fitted). Note that the difference in the slope between the calculated decline curves for 10 MU versus 3 MU (red versus green lines) is dependent on the change in interferon efficacy after dose reduction (inserts). Patients 2-H was calculated to become HCV RNA negative within 14 days if 10 MU INTERFERON was continued (red line figure 2-d), but after dose reduction the actual viral decline curved shifted dramatically due to a strong decline in INTERFERON efficacy (green line).