Even if pre-exposure prophylaxsis (PrEP) only prevented onein two HIV cases and were taken only half the time, it would keep HIVprevalence at current levels among gay men with high-risk behaviors and preventas much as a 5 percent prevalence increase, according to an Australian studyreported by aidsmap. 

The mathematical analysis found that continuous PrEP wouldbe barely cost effective, even though it would significantly lower prevalenceamong high-risk gay men. Intermittent PrEP could be more cost effective, but itwould need to be at least 90 percent effective, which is a higher efficacy ratethan it has currently demonstrated. 

According to the article, PrEP would not be cost effectivefor heterosexuals or gay men without high-risk behaviors. The continuous PrEPscenario would cost $330 million per year with a minimum cost of $35,000 perindividual annually.