Combination HIV prevention: the value and interpretation of mathematical models.

Mathematical models of HIV prevention interventions often provide critical insights related to programmatic design and economic efficiency. One recent dynamic model by Long et al. highlights that a combination prevention approach - with testing, treatment, circumcision, microbicides and PrEP - may decrease transmissions by over 60% and may be very cost-effective in South Africa. In this analysis, the authors introduce the critical concept of joint effectiveness of preventions programs and demonstrate how some programs operate synergistically (HIV screening coupled with early treatment) while others may create redundancies (microbicides coupled with pre-exposure prophylaxis). Whether combination HIV prevention programs perform with additive, multiplicative or maximal effectiveness will be important to consider in anticipation of their combined transmission impact.