Background: A consensus statement released on behalf of the Swiss Federal Commission for HIV/AIDS suggests that people receiving effective antiretroviral therapy-ie, those with undetectable plasma HIV RNA (<40 copies per mL)-are sexually non-infectious. We analysed the implications of this statement at a population level.

Methods: We used a simple mathematical model to estimate the cumulative risk of HIV transmission from effectively treated HIV-infected patients (HIV RNA <10 copies per mL) over a prolonged period. We investigated the risk of unprotected sexual transmission per act and cumulatively over many exposures, within couples initially discordant for HIV status.

Findings: Assuming that each couple had 100 sexual encounters per year, the cumulative probability of transmission to the serodiscordant partner each year is 0.0022 (uncertainty bounds 0.0008-0.0058) for female-to-male transmission, 0.0043 (0.0016-0.0115) for male-to-female transmission, and 0.043 (0.0159-0.1097) for male-to-male transmission. In a population of 10 000 serodiscordant partnerships, over 10 years the expected number of seroconversions would be 215 (80-564) for female-to-male transmission, 425 (159-1096) for male-to-female transmission, and 3524 (1477-6871) for male-to-male transmission, corresponding to an increase in incidence of four times compared with incidence under current rates of condom use.

Interpretation: Our analyses suggest that the risk of HIV transmission in heterosexual partnerships in the presence of effective treatment is low but non-zero and that the transmission risk in male homosexual partnerships is high over repeated exposures. If the claim of non-infectiousness in effectively treated patients was widely accepted, and condom use subsequently declined, then there is the potential for substantial increases in HIV incidence.

Funding: Australian Research Council.