Correspondence (The Lancet 343, 1293-4, May 21,1994)
Home Testing for HIV
Nosh Jamie Robinson, Bertran Auvert
INSERM U88, 91 Boulevard de L Hopital, Paris 75013. France
Dan Mulder
MRC/ODA Programme on AIDS in Uganda, Entebbe, Uganda
Richard Hayes
London School of Hygiene and Tropical Medicine, London, UK
Sir -- Mastro and colleagues [3] (Jan 22, p 204) report estimates the probability of HIV-1 transmission per sexual contact, from female prostitutes to male military conscripts in northern Thailand. Their conservative estimate of the transmission probability, based on all the men, was 0.031 (95% CI 0.025-040). In a subgroup of men not reporting a history of other sexually transmitted diseases (STDs), their estimate was 0.012 (0.006-0.025). They attribute this unexpectedly high value to the possible presence of STDs in female prostitutes (which may have enhanced HIV transmission), and/or so high levels of infectivity among the prostitutes who were likely to be at an early stage of HIV infection.
Frerichs apparently overlooks these explanations and infers that the probability of transmission of HIV between regular partners would be the same as that in prostitute-client contacts. Thus, with a probability 0.031, he estimated that over 90% of initially uninfected regular partners of seropositive persons would acquire infection over 1 year; this is Inconsistent with data from prospective studies in developing countries suggesting seroconversion rates among HIV discordant partners of about 10% per year for both male and female index cases.[1, 2] If it is assumed that couples on average have two sexual contacts per week, then, on the basis of simple probability calculations, this gives an average transmission probability per sexual contact of about 0.001 (over 30 times smaller than the conservative estimate of Mastro et al).
Two important issues should be highlighted. Firstly, estimates of female-to-male transmission probabilities from prospective studies of long-term partners are an order of magnitude lower than estimates derived by Mastro and colleagues in their study in northern Thailand. The higher transmission rate in the Thai study probably indicates conditions associated wish one-off sexual contacts between prostitutes and their clients and is likely to considerably overestimate transmission razes between regular partners. For estimating the spread of HIV infection this is critical.
Secondly, estimates of transmission rates from prospective studies of HIV discordant partners from developing and industrialized countries are similar,[3a] whereas transmission rates during prostitute-client contacts may be much lower in industrialized countries than in developing countries.[4] Lower rates of transmission of HIV from commercial sex in industrialized countries probably indicate more regular use of condoms and lower prevalence of other STDs among prostitutes in these countries than among those in developing countries.[4a] Lack of condom use and a high prevalence of STD in one-off sexual contacts, such as those between prostitutes and their clients, probably create conditions that allow rapid and widespread heterosexual epidemics in developing countries.[5]
1. Kengaya-Kayondo JF, Wagner HU, Malamba S, Nunn AJ, Mulder DW. Risk factors for HIV-1 infection: a study of incident cases in a rural Ugandan population. IX International Conference on AIDS, Berlin, Germany, June, 1993, Abstract WC-C02-2.
2. Serwadda A, Sewankambo N, Stallings R, et al. Dynamics of HIV infection among HIV discordant couples In Rakai, a rural district in Uganda. VII International Conference on AIDS in Africa, Marakesh, Morocco, December, 1993. Abstract Th.RT.031.
3. Longini IM Jr, Clark WS, Habe M, Horsburgh CR Jr. The stages of HIV infection: waiting times and infection transmission probabilities, In: Castillo-Chavez C, ed. Lecture notes in biomathematics: mathematical and statistical approaches to AIDS epidemiology, vol 83, Berlin: Springer Verlag, 1989: 111-37.
3a. Same reference at #3
4. Day S, Ward H, Perrota L. Prostitution and risk of HIV: male partners of female prostitutes. BMJ 1993; 307: 359-61.
4a. Same reference as #4
5. Robinson NJ, Hayes R, Mulder D. Using condoms to prevent transmission of HIV. BMJ 1993; 307: 1007.