A model for HIV-1 transmission amongst men who have sex with men

Published: August 1, 2008

A model for HIV-1 transmission amongst men who have sex with men according to sexual behaviour, optimism-scepticism, and sexual beliefs about plasma HIV-1 load and infectivity

Background: Evidence suggests that HIV-1 sexual risk-behaviour is associated with beliefs about blood plasma HIV-1 load (BPVL) and ART. We present a model to estimate the number of new HIV-1 infections to arise from homosexually active HIV-1 positive men in Sydney according to optimism-scepticism (OS), sexual beliefs (SB), and anal intercourse (AI).

Methods: 119 asymptomatic, HIV-1 positive men receiving or not receiving ART completed questionnaires pertaining to sexual practices, clinical demographics, and OS and SB (using validated scales) in the previous 3 months. Our model is based on a robust Bernoulli process for HIV-1 transmission, as follows: P=(1-p)[1-(1-(1-b)aui)ui(1-(1-b)aur)ur(1-(1-b)api)pi(1-(1-b)apr)pr] where P is the probability that a subject transmitted HIV-1 to his casual partners. p is the probability that a partner is HIV-1 positive, negative or serostatus unknown. b is the risk reduction attributable to OS or SB (total Liekert scores). Sensitivity analysis modelled the effect of varying b . Frequencies of unprotected/protected insertive/receptive AI are ui, ur, pi and pr. a is the HIV-1 transmission probability per act. The expected number of new infections per subject is n=Pxc, where c is the total number of casual partners. The expected total number of new infections is N=S (Pxc).

Results: Most AI (92.5%) was protected with HIV-1 negative partners whereas one-third of AI with partners of unknown serostatus was unprotected. N according to OS and SB was 9.0 and 9.9, respectively, amongst HIV-1 negative partners, cf. 31.2 and 30.8, respectively, amongst partners of unknown serostatus (i.e. thrice more infections).

Conclusions: This is the first mathematical model to predict the number of new HIV-1 infections amongst the casual sex partners of infected men depending on OS and SB. Our model shows that potentially more infections occur amongst men whose HIV-1 serostatus is unknown and highlights the need for interventions that address OS, SB and disclosure of HIV-1 serostatus.

-Abstract available at link below-

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