Poster from CROI 2011

Dobromir Dimitrov*1, M-C Boily2, S Abdool Karim3, and B Mâsse1,4
Fred Hutchinson Cancer Res Ctr, Seattle, WA, US; 2Imperial Coll London, UK; 3Univ of KwaZulu-Natal, Durban, South Africa; and 4Univ of Montreal, Canada


Background:
The role of anal intercourse in the overall heterosexual HIV epidemic remains unclear. However, it may be an important risk factor because the considerably higher risk of HIV infection during unprotected receptive anal intercourse compared to vaginal intercourse. Anal intercourse is widely practiced by heterosexuals in many countries; in Tanzania, 6% of sexually active school pupils reported anal intercourse at their first sexual experience. In Cape Town, 10 to 14% of the study participants reported engaging in anal intercourse over the last 3 months. Different mathematical modeling studies have assessed the potential impact of a vaginal microbicide in heterosexual populations and of a rectal microbicide for homosexuals. However, none have assessed the potential impact of a rectal microbicide in heterosexual population. Our study aims to compare the potential impact of rectal, vaginal, and bi-compartment (i.e. applied vaginally and protective during vaginal and anal intercourse) microbicides to prevent HIV acquisition and transmission in heterosexual populations.

Methods:
Risk equations were used to determine under which conditions a rectal microbicide could be as useful as a vaginal microbicide. A transmission dynamic model was used to assess the population-level impact of the different microbicides in a variety of intervention scenarios and high HIV prevalence settings and to predict the fractions of new HIV infections prevented over fixed time periods.

Results:
Without anal intercourse, a 50% efficacious vaginal microbicide used by 100% of females prevents about 10% and 25% of all new male and female HIV infections over 10 years if adherence is 30% and 75%, respectively. These 10-year infection preventions are reduced by 32% in populations with 10% frequency of receptive anal intercourse, assuming 4-fold increase in transmission risk per receptive anal intercourse (RRRAI). A rectal microbicide could be as effective as a vaginal microbicide in populations with anal intercourse rates ranging from 5% to 20% across a range on RRRAI, assuming similar efficacy and frequency of use of both products. A rectal microbicide has less impact than a vaginal microbicide in populations with <5% anal intercourse, unless it is used more often or is more efficacious than a vaginal microbicide. The 10-year infections prevented of bi-compartment microbicide is 2-fold larger than vaginal microbicide in populations with 10% anal intercourse if RRRAI = 10- and ~6-fold larger than rectal microbicide, in populations with 5% anal intercourse if RRRAI = 4.

Conclusions:
Both rectal microbicide and bi-compartmental microbicide are necessary prevention tools for heterosexual populations engaging, relatively frequently (~10% of sex acts), in anal intercourse.

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