Elsevier

The Lancet

Volume 357, Issue 9263, 14 April 2001, Pages 1149-1153
The Lancet

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Probability of HIV-1 transmission per coital act in monogamous, heterosexual, HIV-1-discordant couples in Rakai, Uganda

https://doi.org/10.1016/S0140-6736(00)04331-2Get rights and content

Summary

Background

The probability of HIV-1 transmission per coital act in representative African populations is unknown. We aimed to calculate this probability overall, and to estimate how it is affected by various factors thought to influence infectivity.

Methods

174 monogamous couples, in which one partner was HIV-1 positive, were retrospectively identified from a population cohort in Rakai, Uganda. Frequency of intercourse and reliability of reporting within couples was assessed prospectively. HIV-1 seroconversion was determined in the uninfected partners, and HIV-1 viral load was measured in the infected partners. Adjusted rate ratios of transmission per coital act were estimated by Poisson regression. Probabilities of transmission per act were estimated by log-log binomial regression for quartiles of age and HIV-1 viral load, and for symptoms or diagnoses of sexually transmitted diseases (STDs) in the HIV-1-infected partners.

Results

The mean frequency of intercourse was 8·9 per month, which declined with age and HIV-1 viral load. Members of couples reported similar frequencies of intercourse. The overall unadjusted probability of HIV-1 transmission per coital act was 0·0011 (95% CI 0·0008–0·0015). Transmission probabilities increased from 0·0001 per act at viral loads of less than 1700 copies/mL to 0·0023 per act at 38 500 copies/mL or more (p=0·002), and were 0·0041 with genital ulceration versus 0·0011 without (p=0·02). Transmission probabilities per act did not differ significantly by HIV-1 subtypes A and D, sex, STDs, or symptoms of discharge or dysuria in the HIV-1-positive partner.

Interpretation

Higher viral load and genital ulceration are the main determinants of HIV-1 transmission per coital act in this Ugandan population.

Introduction

The probability of HIV transmission per sexual act of vaginal intercourse, or infectivity (γ), has been estimated from prospective studies of HIV-discordant partners or male contacts with prostitutes.1 Published estimates of transmission probabilities per act vary from γ=0·0001 to 0·0014 in US and European studies of discordant couples,2, 3, 4, 5, 6 and to γ=0·002 in Thai couples.7 However, higher transmission probabilities (γ=0·056–0·100 per act) have been reported among men who had contacts with prostitutes in Thailand8 and Kenya.9 The higher transmission probabilities associated with commercial sex might be attributable to the presence of other sexually transmitted diseases (STDs), which are thought to increase infectivity of or susceptibility to HIV,2, 8, 9 or to possible errors in reported contact frequency. Other factors that increase transmission include lower CD4 counts or AIDS in the HIV-positive index partner, and anal intercourse.2, 5, 8, 9 Male-to-female HIV transmission is usually more efficient than female-to-male transmission in US and European populations, but the small numbers of HIV-positive female index partners limit conclusive sex-specific estimates of transmission probabilities per sex act.2, 3, 4, 5, 10, 11

There are no data on per-contact probability of transmission from representative heterosexual couples in sub-Saharan Africa, and there is little information on the efficiency of transmission associated with HIV-1 viral subtypes.2 The eastern and southern African HIV epidemic is predominantly caused by HIV-1 subtypes A, C, and D, and the rapid progression of the epidemic in these parts of Africa might, in part, be associated with greater infectivity of these HIV-1 subtypes.12 We assessed this possibility by examining transmission probabilities for subtypes A and D in Uganda. Conversely, some have postulated that the slower evolution of the HIV epidemic in western Africa might be due to the higher proportion of less transmissible recombinant HIV-1 subtypes in that region.13 To address these issues, we estimated the per-contact probability of HIV-1 transmission among monogamous, heterosexual HIV-1-discordant couples in rural Rakai District, Uganda, in which the HIV-1 epidemic is due to viral subtypes A and D. Estimates of HIV transmission probabilities rely on the accuracy of reports of the frequency of sexual intercourse,1 and previous studies have not assessed the reliability of such data. Therefore, we also assessed the intracouple reliability of reports of sexual frequency and examined factors such as age, sex, and viral load, which might be associated with frequency of intercourse.

Section snippets

Participants

Data were collected between November, 1994, and October, 1998, in a community-randomised trial of STD control for AIDS prevention in the rural district of Rakai.14 The original trial enrolled 15 127 individuals aged 15–59 years, who were followed up in their homes every 10 months.

Among the 15 127 participants, 8898 were married or in a consensual union, and information was requested on the identity of their partners. However, to maintain confidentiality, linkage of members of couples was not

Results

Among the 174 HIV-discordant monogamous couples, the HIV-1-positive partners had a median age of 29·0 years (mean 30·2), and a median viral load of 12 476 copies/mL. In 97 couples, the male partner was HIV-1-positive, 17 of whom transmitted the virus to their wives (17·5%). In 77 couples, the woman was the HIV-1-positive partner, 21 of whom transmitted HIV-1 to their husbands (27·3%). HIV-1-infected men were significantly older than infected women (mean ages 34·0 years and 27·1, respectively;

Discussion

The overall probability of transmission per coital act of 0·0011 in the Rakai population is within the range of transmission probabilities per act (0·0001–0·0020), reported from prospective studies of European, north American, and Thai heterosexual couples.2, 3, 4, 5, 6, 7 Condom use was low in the monogamous Ugandan couples, despite the fact that condoms and counselling were offered free of charge and promoted by the project, 56% of HIV-1-positive partners in these discordant relationships had

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