To do this they used two different calculations for the relationship between viral load and transmission, derived from studies with heterosexuals in Uganda and Zambia.
The first calculation has been widely used by other researchers. In it, each log increase in viral load is assumed to increase transmission 2.45-fold. While this 2.45-fold relationship is thought to be accurate for viral loads between 400 and 10,000 copies/ml, Baggaley and colleagues believe that it overestimates transmission both at lower and higher viral loads.
The second, more complex, calculation reflects transmission being extremely rare at low viral loads and also transmission rates being pretty constant at higher viral loads.
Using the first method, the HIV transmission risk for unprotected receptive anal intercourse is 0.06%, which is 96% lower than without treatment. However using the second method, the predicted transmission risk would be 0.0011%, which is 99.9% lower than without treatment.