  Anti-retroviral drugs can reduce the infectiousness of people living with HIV
by about 96%--treatment as prevention or TasP--and can reduce the risk of being
infected by an HIV positive person by about 70%--pre-exposure prophylaxis or
PrEP--raising the prospect of using anti-retroviral drugs to stop the epidemic
of HIV. The question as to which is more effective, more affordable and more
cost effective, and under what conditions, continues to be debated in the
scientific literature. Here we compare TasP and PreP in order to determine the
conditions under which each strategy is favourable. This analysis suggests that
where the incidence of HIV is less than 5% or the risk-reduction under PrEP is
less than 50%, TasP is favoured over PrEP; otherwise PrEP is favoured over
TasP. The potential for using PreP should therefore be restricted to those
among whom the annual incidence of HIV is greater than 5% and TasP reduces
transmission by more than 50%. PreP should be considered for commercial sex
workers, young women aged about 20 to 25 years, men-who-have-sex with men, or
intravenous drug users, but only where the incidence of HIV is high.
