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University of Washington
Fred Hutchinson Cancer Research Center, Seattle
University of Nairobi, Nairobi, Kenya
Potterat [1] raises several questions concerning the precision of the data used in our recent article on female-to-male HIV-1 infectivity among Kenyan truck drivers [2]. First, Potterat asks how questions regarding sexual behavior were phrased and, as a result, whether sexual activity with male partners might have been overlooked. For the purposes of the infectivity model reported in our article, we used data from questions that specifically addressed sexual activity with wives, casual partners, and prostitutes. In the interview, "casual partners" were defined as girlfriends or other casual female partners. At each visit, participants were also asked about sexual activity with male partners. As we report in the Results section of our article, no participants reported sex with men, at any follow-up visit. All sexual behavior data were collected using individual, confidential, face-to-face interviews conducted by trained research staff. Although it is possible that taboos about homosexual behavior may have led to underreporting, it is notable that, at their enrollment visit, 3 men reported a history of sex with men [3], suggesting that our data-collection method was able to elicit candid responses to this question.
Second, Potterat asks whether data on anal intercourse were sought. Again, we refer to the Results section of our article, in which we note that only 5 men (<1%) reported ever having had anal sex with a woman. Among a group of female prostitutes in Mombasa, Kenya, during this same period, <1% reported having had anal sex [4], suggesting that this practice was uncommon in this population. Thus, penile-vaginal intercourse was the dominant sexual activity for participants in our study.
Third, Potterat expresses concern that parenteral exposures, such as to HIV-1contaminated medical injection equipment, may have confounded our results. Specifically, he questions whether uncircumcised men were more likely to acquire a genital infection than were circumcised men and, thus, more likely to have received injections for treatment. As is detailed in our earlier study of this cohort [5], the incidences of gonococcal and nongonococcal urethritis during follow-up did not differ significantly between uncircumcised and circumcised men. Genital ulcer disease occurred more frequently among those who were uncircumcised [5], but the relationship between lack of male circumcision and HIV-1 acquisition was virtually unchanged when adjusted for genital ulcer disease, and genital ulcer disease was not significantly associated with HIV-1 acquisition, after circumcision status was taken into consideration. Thus, potential injections for treatment of genital ulcer disease did not contribute measurably to HIV-1 acquisition in this cohort. The role of unsafe injections in the African HIV-1 epidemic has been analyzed and reviewed in detail elsewhere [6, 7], and those studies confirmed that parenteral exposures are not the principal route of transmission.
The results of our study are consistent with those of previous studies that have repeatedly demonstrated that a lack of male circumcision is associated with an increased risk of HIV-1 acquisition [8]. Our estimates of HIV-1 infectivity per act of penile-vaginal intercourse are higher than those reported in studies of monogamous HIV-1serodiscordant couples but lower than those reported in studies of sexual activity with prostitutes [9]. This suggests that our infectivity estimates reflect the sexual behavior of the men in our study, who were often married but who also had relationships with casual partners and prostitutes. Our infectivity estimates should be considered valid for populations in which multiple, concurrent partnerships are common, such as is found in much of Africa. Moreover, they reflect the fact that heterosexual vaginal sex is the dominant mode of HIV-1 transmission in Africa [10].
References
1. Potterat JJ. Estimating female-to-male infectivity of HIV-1 in Kenya: potential threats to validity . J Infect Dis 2005; 191:21545 (in this issue). First citation in article
2. Baeten JM, Richardson BA, Lavreys L, et al. Female-to-male infectivity of HIV-1 among circumcised and uncircumcised Kenyan men. J Infect Dis 2005; 191:54653. First citation in article
3. Baeten JM, Chohan BH, Lavreys L, et al. Correlates of human herpesvirus 8 seropositivity among heterosexual men in Kenya. AIDS 2002; 16:20738. First citation in article
4. Martin HL Jr, Nyange PM, Richardson BA, et al. Hormonal contraception, sexually transmitted diseases, and risk of heterosexual transmission of human immunodeficiency virus type 1. J Infect Dis 1998; 178:10539. First citation in article
5. Lavreys L, Rakwar JP, Thompson ML, et al. Effect of circumcision on incidence of human immunodeficiency virus type 1 and other sexually transmitted diseases: a prospective cohort study of trucking company employees in Kenya. J Infect Dis 1999; 180:3306. First citation in article
6. Schmid GP, Buve A, Mugyenyi P, et al. Transmission of HIV-1 infection in sub-Saharan Africa and effect of elimination of unsafe injections. Lancet 2004; 363:4828. First citation in article
7. Walker PR, Worobey M, Rambaut A, Holmes EC, Pybus OG. Sexual transmission of HIV in Africa: other routes of infection are not the dominant contributor to the African epidemic. Nature 2003; 422:679. First citation in article
8. Bailey RC, Plummer FA, Moses S. Male circumcision and HIV prevention: current knowledge and future research directions. Lancet Infect Dis 2001; 1:22331. First citation in article
9. Baeten JM, Overbaugh J. Measuring the infectiousness of persons with HIV-1: opportunities for preventing sexual HIV-1 transmission. Curr HIV Res 2003; 1:6986. First citation in article
10. UNAIDS/WHO. AIDS epidemic update 2004. Available at: http://www.unaids.org/wad2004/EPI_1204_pdf_en/EpiUpdate04_en.pdf. Accessed 30 December 2004. First citation in article