Share this article on:

Data are lacking for quantifying HIV transmission risk in the presence of effective antiretroviral therapy

Wilson, David P

doi: 10.1097/QAD.0b013e32832d871b
Epidemiology and social: EDITORIAL COMMENT

National Centre in HIV Epidemiology and Clinical Research, University of New South Wales, Sydney, Australia.

Received 21 April, 2009

Accepted 29 April, 2009

Correspondence to A/Professor David P. Wilson, National Centre in HIV Epidemiology and Clinical Research, University of New South Wales, Level 2, 376 Victoria Street Darlinghurst, Sydney, NSW 2010, Australia. Tel: +61 2 9385 0900; fax: +61 2 9385 0920; e-mail:

The current era of HIV/AIDS epidemics is one in which highly effective combination antiretroviral therapy (ART) is being used to treat a high proportion of HIV-infected individuals who are diagnosed with their infection in the developed world, and universal treatment access is becoming closer to reality for treatment-eligible individuals in the developing world. Therefore, it is of greater importance to understand the magnitude of HIV transmission risk in the presence of ART. In this issue of AIDS, Attia et al. [1] report results from a systematic review of the literature and meta-analysis of the risk of HIV through unprotected sexual intercourse according to viral load, with or without the presence of ART. A review of this kind is timely, particularly as the role of ART in prevention is currently a hotly debated topic [2–5]. Several longitudinal and cross-sectional studies of HIV discordant couples have reported on transmission risk against viral load, with or without ART. Until now, however, no study had carried out a transparent and comprehensive review and analysis of the body of evidence available.

Attia et al. [1] established that very few studies, with relatively small numbers of person-years of follow-up, have reported HIV transmission in discordant partnerships in which viral load was reported in the presence of ART. The overall HIV transmission rate in the presence of ART, irrespective of viral load, was calculated to be approximately 0.5 per 100 person-years. Considerably more studies with substantially longer follow-up times have been published on transmission rates according to viral load in the absence of ART; Fig. 1 demonstrates the findings of Attia et al. [1] regarding transmission risk versus viral load. There is a predictable relationship between HIV viremia and transmission rates for viral levels above approximately 400 copies per ml (Fig. 1). However, in the range of viral levels relevant to suppressive therapy, there is large uncertainty in the transmission rate. There may well be a sharp threshold level below which transmission becomes highly improbable. But it seems more apparent that studies conducted to date have been insufficiently powered to measure transmission rates for lower viral loads, and there is likely to be a continuous association between viremia and transmission risk. Available data on transmission events for viral load less than 400 copies/ml led to a transmission rate estimate of 0.16 [0.02–1.13, 95% confidence interval (CI)] per 100 person-years. Mathematical curve fits of the data diverge at low viral levels, due to insufficient data, and extrapolation of standard regression models (linear relationships on the log-log scale) is inappropriate for estimating transmission risk for low viral loads. In summary, as highlighted by Attia et al. [1], data are currently lacking for quantifying transmission risk for low HIV levels or in the presence of suppressive ART. Evidence is also currently not available for distinguishing transmission rates under ART according to condom use, presence or absence of other sexually transmitted infections, or direction and mode of sexual intercourse (insertive or receptive vaginal or anal sex).

Fig. 1

Fig. 1

Although there is large uncertainty in transmission risk in the presence of effective ART, greater precision can be obtained. Results from the prospective cohort study led by Cohen [6] are eagerly anticipated to fill the knowledge gap. Accumulating case reports of transmission events from HIV-infected partners with low viral loads may also assist in determining a transmission threshold, if it exists. Transmission rates (and threshold levels) could also be expected to depend not only on viral load but also on whether or not ART is present and which combination antiretroviral regimen is used. ART regimens penetrate genital fluids to differing degrees; the correlation between plasma viral load and viral load in semen, cervicovaginal, or rectal fluids is not perfect and differs by drug class and regimen [7–9].

ART does decrease HIV-RNA levels in blood and semen [7–9], strongly suggesting that effective treatment will also lower the risk of transmission from a person infected with HIV [10]. But the magnitude of reduction is still not well known. Mathematical models have been used to predict the effect of ART on epidemics. Modeling is a useful tool for linking individual-level parameters with population-level outcomes if appropriate assumptions are made. Models have made assumptions of ART reducing transmission ranging from two to 100 times [3,4,11,12]. Attia et al. [1] calculated that ART reduces transmission rates by approximately 30-fold (5.71 per 100 person-years to 0.19 per 100 person-years), irrespective of viral load. The use of mathematical models in the future to forecast HIV epidemics under conditions of high ART coverage will depend on the accuracy of estimates of transmission risk in the presence of ART. Paradoxically, in an era of increasing rates of successful treatment that achieves viral suppression and lowers infectiousness, many settings are experiencing increases in HIV notifications, particularly in the developed world [13–17]. It is of high importance and relevance to counseling in clinical practice as well as modeling and public health evaluation that the international HIV research community works toward elucidating the actual risk of transmission under effective ART.

Back to Top | Article Outline


1. Attia S, Egger M, Müller M, Zwahlen M, Low N. Sexual transmission of HIV according to viral load and antiretroviral therapy: systematic review and meta-analysis. AIDS 2009; 23:1397–1404.
2. Vernazza P, Vernazza P, Hirschel B, Bernasconi E, Flepp M. HIV-positive individuals without additional sexually transmitted diseases (STD) and on effective anti-retroviral therapy are sexually non-infectious HIV-positive individuals without additional sexually transmitted diseases (STD) and on effective anti-retroviral therapy are sexually non-infectious. Bull des Médecins Suisses 2008; 89:165–169.
3. Wilson DP, Law MG, Grulich AE, Cooper DA, Kaldor JM. Relation between HIV viral load and infectiousness: a model-based analysis. Lancet 2008; 372:314–320.
4. Granich RM, Gilks CF, Dye C, De Cock KM, Williams BG. Universal voluntary HIV testing with immediate antiretroviral therapy as a strategy for elimination of HIV transmission: a mathematical model. Lancet 2008; 373:48–57.
5. DeCock KM, Gilks CF, Lo YR, Guerma T. Can antiretroviral therapy eliminate HIV transmission? Lancet 2009; 373:7–9.
6. Cohen MS, Bollinger RC, Celentano D, Chariyalertsak S, Grinstejn B, Hakim J, Cohen MS, et al. HPTN 052. A randomized trial to evaluate the effectiveness of antiretroviral therapy plus HIV primary care versus HIV primary care alone to prevent the sexual transmission of HIV-1 in serodiscordant couples. Available at [accessed April 2009].
7. Vernazza PL, Gilliam BL, Dyer J, Fiscus SA, Eron JJ, Frank AC, Cohen MS. Quantification of HIV in semen: correlation with antiviral treatment and immune status. AIDS 1997; 11:987–993.
8. Vernazza PL, Troiani L, Flepp MJ, Cone RW, Schock J, Roth F, et al. Potent antiretroviral treatment of HIV-infection results in suppression of the seminal shedding of HIV. The Swiss HIV Cohort Study. AIDS 2000; 14:117–121.
9. Zhang H, Dornadula G, Beumont M, Livornese L Jr, Van Uitert B, Henning K, Pomerantz RJ. Human immunodeficiency virus type 1 in the semen of men receiving highly active antiretroviral therapy. N Engl J Med 1998; 339:1803–1809.
10. Porco TC, Martin JN, Page-Shafer KA, Cheng A, Charlebois E, Grant RM, Osmond DH. Decline in HIV infectivity following the introduction of highly active antiretroviral therapy. AIDS 2004; 18:81–88.
11. Blower SM, Gershengorn HB, Grant RM. A tale of two futures: HIV and antiretroviral therapy in San Francisco. Science 2000; 287:650–654.
12. Law MG, Prestage G, Grulich A, Van de Ven P, Kippax S. Modelling the effect of combination antiretroviral treatments on HIV incidence. AIDS 2001; 15:1287–1294.
13. National Centre in HIV Epidemiology and Clinical Research (NCHECR). HIV/AIDS, viral hepatitis and sexually transmissible infections in Australia: annual surveillance report; 2006. Sydney, Australia: NCHECR; 2006.
14. Fisher M, Pao D, Murphy G, Dean G, McElborough D, Homer G, Parry JV. Serological testing algorithm shows rising HIV incidence in a UK cohort of men who have sex with men: 10 years application. AIDS 2007; 21:2309–2314.
15. Dukers NH, Spaargaren J, Geskus RB, Beijnen J, Coutinho RA, Fennema HS. HIV incidence on the increase among homosexual men attending an Amsterdam sexually transmitted disease clinic: using a novel approach for detecting recent infections. AIDS 2002; 16:F19–F24.
16. The UK Collaborative Group for HIV and STI Surveillance. Mapping the issues. HIV and other sexually transmitted infections in the United Kingdom in 2004. London: Health Promotion Agency, Centre for Infections; 2005.
17. Grulich AE, Kaldor JM. Trends in HIV incidence in homosexual men in developed countries. Sex Health 2008; 5:113–118.

antiretroviral therapy; HIV transmission risk; viral load

© 2009 Lippincott Williams & Wilkins, Inc.