I heard of a recent study that said the risk of infection among serodiscordant couples is somewhere between 1 in 450 and 1 in 700 per sexual contact. Does this mean that the risk of getting infected when you use condoms with a positive person is 1 in a million? Thanks.
The study to which you refer was recently presented at the 8th Conference on Retroviruses and Opportunistic Infections by investigators from Johns Hopkins (see The Body's coverage of this presentation under <A HREF="http://www.thebody.com/confs/retro2001/pavia10.html:>Epidemiology and Infection Control). The research team followed 174 monogamous, serodiscordant (one partner is HIV infected and the other is not) heterosexual couples in Uganda to determine the probability of HIV transmission for each act of vaginal intercourse among people living in sub-Saharan Africa. This study tests the theory that strains of HIV in Africa are more easily transmittable than other strains in the world. The couples had an average of 9 sexual encounters per month, and 38 partners became infected during the study. 18% of the men and 28% of the women became infected by their HIV-infected partners. The risk for infection per coital act was 1.7 per thousand/1 in 500.
Based on the study's findings, the investigators surmised that HIV strains in Africa are not more easily transmittable. We probably see men getting infected in Africa as much as women when compared with U.S. infection rates because there are significantly more women infected in Africa than in the U.S. That means that a male in Africa is more likely to be exposed to HIV when having unprotected sex with a female than elsewhere. Other factors--not being circumcised, genital ulcers, and viral load (this being the strongest predictor)--probably increased the likelihood of transmission.
It makes sense that using condoms would decrease the odds of infection considerably, but you cannot determine the exact odds based on this study. You need to look at research that specifically examines infection rates among couples who are using condoms. Read my response to Condom Failure for information on condom efficacy.