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Thursday, 3rd of September 2009 Print

 Writing in The Lancet Infectious Diseases, Boilly and her colleagues review
 the evidence, from developed and developing countries, on the risk of HIV
 acquisition per heterosexual act. Although the risk of HIV acquisition is
 generally less than 1 percent per act,  receptive anal intercourse was
 associated with higher risk; so was intact foreskin in the male; so were
 the early and the late stages of HIV infection in the seropositive
 Abstract of their article is below; full text is at
 Their review confirms the earlier work of Powers and colleagues, available
 on the Internet at
 In their summary, Powers and colleagues express reservations about the use
 of the often cited value of 0.001.
 ‘Studies of cumulative HIV incidence suggest that cofactors such as genital
 ulcer disease, HIV disease stage, and male circumcision influence HIV
 transmission; however, the heterosexual infectivity of HIV-1 is commonly
 cited as a fixed value (approximately 0·001, or one transmission per 1000
 contacts). . . . Infectivity estimates were very heterogeneous, ranging
 from zero transmissions after more than 100 penile-vaginal contacts in some
 serodiscordant couples to one transmission for every 3·1 episodes of
 heterosexual anal intercourse. . . . A single value for the heterosexual
 infectivity of HIV-1 fails to reflect the variation associated with
 important cofactors. The commonly cited value of 0·001 was estimated among
 stable couples with low prevalences of high-risk cofactors, and represents
 a lower bound. Cofactor effects are important to include in epidemic
 models, policy considerations, and prevention messages.’
 Good reading.
 Bob Davis
 The Lancet Infectious Diseases, Volume 9, Issue 2, Pages 118 - 129,
 February 2009
 Heterosexual risk of HIV-1 infection per sexual act: systematic review and
 meta-analysis of observational studies
 Original Text
 Dr Marie-Claude Boily PhD a c d , Rebecca F Baggaley PhD a, Lei Wang PhD e,
 Benoit Masse PhD e, Richard G White PhD b, Prof Richard J Hayes DSc b, Prof
 Michel Alary PhD c d
 We did a systematic review and meta-analysis of observational studies of
 the risk of HIV-1 transmission per heterosexual contact. 43 publications
 comprising 25 different study populations were identified. Pooled
 female-to-male (0·04% per act [95% CI 0·01—0·14]) and male-to-female (0·08%
 per act [95% CI 0·06—0·11]) transmission estimates in high-income countries
 indicated a low risk of infection in the absence of antiretrovirals.
 Low-income country female-to-male (0·38% per act [95% CI 0·13—1·10]) and
 male-to-female (0·30% per act [95% CI 0·14—0·63]) estimates in the absence
 of commercial sex exposure (CSE) were higher. In meta-regression analysis,
 the infectivity across estimates in the absence of CSE was significantly
 associated with sex, setting, the interaction between setting and sex, and
 antenatal HIV prevalence. The pooled receptive anal intercourse estimate
 was much higher (1·7% per act [95% CI 0·3—8·9]). Estimates for the early
 and late phases of HIV infection were 9·2 (95% CI 4·5—18·8) and 7·3 (95% CI
 4·5—11·9) times larger, respectively, than for the asymptomatic phase.
 After adjusting for CSE, presence or history of genital ulcers in either
 couple member increased per-act infectivity 5·3 (95% CI 1·4—19·5) times
 versus no sexually transmitted infection. Study estimates among
 non-circumcised men were at least twice those among circumcised men.
 Low-income country estimates were more heterogeneous than high-income
 country estimates, which indicates poorer study quality, greater
 heterogeneity of risk factors, or under-reporting of high-risk behaviour.
 Efforts are needed to better understand these differences and to quantify
 infectivity in low-income countries.
 a Department of Infectious Disease Epidemiology, Faculty of Medicine,
 Imperial College, London, UK
 b Department of Epidemiology and Population Health, London School of
 Hygiene and Tropical Medicine, London, UK
 c Population Health Research Unit, Hôpital du Saint-Sacrement, Centre
 Hospitalier, University of Quebec, Quebec City, Quebec, Canada
 d Department of Social and Preventive Medicine, Faculty of Medicine, Laval
 University, Quebec City, Quebec, Canada
 e Statistical Center for HIV/AIDS Research and Prevention, Fred Hutchinson
 Cancer Research Center, Seattle, WA, USA
 Correspondence to: Dr Marie-Claude Boily, Department of Infectious Disease
 Epidemiology, Imperial College, Norfolk Place, London N1 3LG, UK