We analysed the risk of HIV transmission per 100 person-years of follow up because we only identified one study reporting on HIV transmission per coital act and stratified by viral load [15,16]. Data about coital frequency and transmission events were collected from the same population of untreated HIV-infected individuals in Rakai, Uganda. The estimated probability of HIV transmission per coital act, after controlling for age, ranged from 0.0001 when viral load was below 1700 copies/ml (sexual intercourse 10.4 times per month) to 0.0023 when viral load was greater than 38 500 copies/ml (sexual intercourse 7.9 times per month) .
Five studies included couples in which the HIV-seropositive partners used antiretroviral therapy, with 1098 person years of follow-up [10,11,18,23,24] (Table 1). One study reported specific antiretroviral regimens . We did not identify any studies that reported on both viral load and all microbiologically diagnosed sexually transmitted infections.
The overall HIV transmission risk from antiretroviral-treated patients to heterosexual partners, irrespective of viral load and other sexually transmitted infections, was 0.46 (95% CI 0.19–1.09) per 100 person-years, based on five episodes of HIV seroconversion (Fig. 2) [10,11,18,23,24]. Information on the lowest measured viral load at which transmission had occurred while taking antiretroviral therapy was not available for either of the studies in which new HIV infections occurred [18,23]. In the two studies with information stratified according to viral load there were no reported episodes of HIV transmission from HIV seropositive people with undetectable viral load in 291 person years of follow-up (upper 97.5% confidence limit 1.27 per 100 person years) [10,11]. Three studies did not report on associations between HIV transmission and other sexually transmitted infections [18,23,24]. Melo et al.  and Castilla et al.  reported no association with the infections assessed in their studies (Table 1).
Ten studies included HIV seropositive people not receiving antiretroviral therapy with 9998 person years of follow up [9–14,17,21,23,24]. The overall HIV transmission rate, irrespective of viral load category and sexually transmitted infections, was 5.64 (95% CI 3.28–9.70) per 100 person years (Fig. 2). Amongst people with viral load below 400 copies/ml, irrespective of sexually transmitted infections, the transmission rate was 0.16 (95% CI 0.02–1.13) per 100 person years, based on one episode of HIV transmission in six studies [9,11–14,17]. The transmission rate increased with increasing viral load to 9.03 (95% CI 3.87–21.09) per 100 person years amongst individuals with viral load at least 50 000 copies/ml (Fig. 2).
The lowest measured viral load values around the time of HIV-transmission events were available for seven studies (Table 1). Three values, all in untreated individuals, were below 1500 copies/ml: at 362 copies/ml (Castilla J, 17 July 2008, personal communication), 600 copies/ml (Ragni M, 21 July 2008, personal communication) and 1497 copies/ml .
This systematic review did not identify any study from which the risk of HIV transmission per act of unprotected sexual intercourse amongst persons with suppressed viraemia following ART could be quantified directly. The available studies found no episodes of HIV transmission in discordant heterosexual couples if the HIV-infected partner was treated with ART and had a viral load below 400 copies/ml, but the data were also compatible with one transmission per 79 person-years. There were insufficient data to stratify rates according to the presence or absence of sexually transmitted infections, use of condoms, direction of transmission, or practise of vaginal or anal intercourse. The comparison of overall rates in patients on ART and not on ART nevertheless indicate that heterosexual transmission was reduced by 92%, from 5.64 to 0.46 per 100 person-years. Of note, our review did not identify any study with data on ART and transmission risk in homosexual men.
The main strengths of this study were that we searched systematically for published and unpublished literature and attempted to quantify statistical uncertainty around the transmission rate. Additional information from several authors allowed us to combine data in consistent viral load categories to increase the precision of estimated transmission risks [10–13] and to report the minimum viral load at which HIV transmission occurred. The main limitations of the study relate to the lack of data that could be combined statistically. Four included studies were only available as conference abstracts with limited details [18,21,23,24]. Precision was also limited by small or zero numbers of events in each viral load category and short follow-up times. There are recognized difficulties in obtaining confidence intervals when no events have been observed . The interpretation of the lower limit of zero and upper 97.5% limit obtained using exact Poisson distribution probabilities differs from the standard 95% confidence interval. However, they demonstrate the uncertainty about the true HIV transmission rate by describing a range of values for the true quantity of interest that are compatible with the observed data. Describing the likelihood function about the true value of the parameter is an alternative but the results obtained would not alter our conclusion.
We found no direct evidence that, as stated by the Swiss Federal AIDS commission , the HIV transmission risk through unprotected sexual intercourse from an infected individual taking ART consistently under medical supervision, with blood viral load below 40 copies/ml and without any other sexually transmitted infection was ‘much lower than one per 100 000 acts of sexual intercourse.’ We found that there is considerable uncertainty about this risk: first, although there were no observed episodes of HIV transmission from people with undetectable viral load on highly active antiretroviral therapy, data are compatible with one new HIV infection for every 79 person-years of follow-up (one per 7900 sex acts if the yearly average is 100 contacts  and transmission probability is constant). Second, episodes of HIV transmission were found to have occurred at viral load levels lower than reported in earlier studies . There might therefore be no transmission threshold or a lower threshold than previously believed [5,26].
Mathematical models have been developed to predict the effects of antiretroviral therapy on HIV transmission but variability in assumptions based on epidemiological and biological data makes them difficult to interpret . The reduction in HIV transmissibility due to antiretroviral therapy includes estimates from two to 100 times , two to 10 times , and 100 times . Our meta-analysis should be useful in this context and inform future modelling studies.
There is also uncertainty about the role of sexually transmitted infections. Focusing on ulcerative conditions and symptoms as a proxy for lower genital tract inflammation  is problematic. First, symptoms in women correlate poorly with clinical signs of inflammation or diagnosed infections . Second, HIV transmission appears to be enhanced by bacterial vaginosis , a vaginal infection characterized by an absence of inflammation . Third, sexual transmission of herpes simplex virus, the most common cause of genital ulcer disease in many countries, can occur during asymptomatic virus shedding . Suppressing clinical recurrences with acyclovir does not reduce the risk of HIV transmission [34,35]. Furthermore, adherence to recommendations for regular testing for sexually transmitted infections in HIV-infected people would have to improve from current levels .
The risk of HIV transmission from people on highly active antiretroviral therapy is likely to be very low but it is nevertheless important that statements on transmission risk are based on thorough evaluation of the available data [37,38]. The need for systematic searches and clear documentation about the design, quality and consistency of evidence, and the availability or absence of direct evidence to address important clinical and public health questions is well recognized . The users of recommendations can then distinguish between statements based on appraisal of evidence by experts and those based on systematic methods. The results of our systematic review show where there is a lack of direct evidence and where further research is required.
Greater precision about the HIV transmission rate per sexual act on highly active antiretroviral therapy can be obtained from empirical studies. An upper 95% confidence interval of one in 100 000 per unprotected sex acts would be obtained if the observed HIV transmission rate were one in 550 000 sex acts. This is equivalent to 5500 person years of observation with an average of 100 unprotected sex acts per year, or 1100 couples followed for 5 years having unprotected sex and free of sexually transmitted infections, assuming a constant transmission probability and each act as an independent event. An ongoing randomized trial to follow 1750 HIV serodiscordant heterosexual couples for a median of 5.75 years with the infected partner receiving highly active antiretroviral therapy will help to provide this information . Studies to determine HIV transmissibility through insertive and receptive anal intercourse when viraemia is fully suppressed are needed to provide direct evidence for men who have sex with men. The implications of differences between antiretroviral agents in drug levels in plasma and genital tract, and of intermittent viral ‘blips’ also need to be clarified .
We are grateful to Jesús Castilla, Jorge del Romero, Kenrad Nelson, Margaret Ragni, and Breno Santos who provided additional data from their studies that contributed to the results presented in this paper. S.A. was supported by a ThinkSwiss! Fellowship from the Swiss Government.
Author contributions: S.A. did the literature searches, study selection, data extraction, and wrote the first draft; M.E. obtained funding, designed and supervised the study, and revised the paper; M.M. contributed to study selection, data extraction, and revision of the paper, M.Z. provided statistical advice and revised the paper; N.L. designed and supervised the study, conducted the analysis, and revised the paper.
Conflicts of interest: N.L. became a member of the Swiss Federal AIDS Commission on 1 January 2008. M.Z. was employed at the Swiss Federal Office of Public Health from 1998–2000 and head of the office of the Swiss Federal AIDS Commission at the Swiss Federal Office of Public Health from 1988–1989. The views expressed here are those of the authors.
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