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Quantifying sexual exposure to HIV within an HIV-serodiscordant relationship: development of an algorithm

Fox, Juliea; White, Peter Jb,c; Weber, Jonathand; Garnett, Geoff Pc; Ward, Helene; Fidler, Sarahc

doi: 10.1097/QAD.0b013e328344fe4a
Epidemiology and Social

Background: The risk of acquiring HIV from a single sexual contact varies enormously reflecting biological and behavioural characteristics of both infected and uninfected partners. Accurate information on HIV transmission risk is required to construct evidence-based risk reduction practices for individuals, to direct the provision of prevention strategies at the population level, and enable the definition, quantification and comparison of true exposure in individuals termed ‘exposed uninfected’ within clinical trials.

Methods: Following a systematic review of current literature on HIV transmission estimates, an HIV risk score was developed, incorporating weighted risk factors into a Bernoulli mathematical model, allowing quantification of overall risk of HIV acquisition within HIV-serodiscordant partnerships.

Results: The HIV risk score enumerates the relative risk of HIV acquisition from HIV-positive partners incorporating the type and frequency of specific sex acts, the index case HIV plasma viral load and stage of disease, and the presence of genital ulcer disease in either partner and pregnancy, HSV-2 seropositivity, and circumcision status (men only) in the HIV-negative partner.

Conclusion: Key determinants of HIV exposure risk can be incorporated into a mathematical model in order to quantify individual relative risks of HIV acquisition. Such a model can facilitate comparisons within clinical trials of exposed uninfected individuals and facilitate interventions to reduce HIV transmission.

aDepartment of HIV, Faculty of Medicine, Guys and St Thomas' NHS Trust/Kings College London, UK

bModelling & Economics Unit, Health Protection Agency, UK

cDepartment of Infectious Disease Epidemiology, MRC Centre for Outbreak Analysis & Modelling, Imperial College Faculty of Medicine, UK

dDepartment of Genitourinary Medicine and Infectious Disease, UK

eDepartment of Infectious Disease Epidemiology, Faculty of Medicine, Imperial College London, London, UK.

Received 2 December, 2009

Revised 13 December, 2010

Accepted 25 January, 2011

Correspondence to Julie Fox, Department of HIV, Faculty of Medicine, Guys and St Thomas' NHS Trust/Kings College London, London, UK. E-mail:

© 2011 Lippincott Williams & Wilkins, Inc.