Australian scientists have warned HIV-positive people that anti-retroviral drugs do not eliminate the need for condoms.
A report earlier this year that said people correctly using antiretroviral drugs might not transmit the virus to their partner during sex.
Four respected HIV researchers had issued a statement on behalf of the Swiss Federal Commission for HIV/AIDS in January this year saying HIV-positive people with undetectable levels of virus in their blood and no other genital infections could not transmit the virus to their HIV-negative partners through sexual contact.
The report was criticised as it concentrated on heterosexual serodiscordant couples, not on male homosexual couples, where the risk of sexual transmission is higher.
(Serodiscordant is a term used to describe a couple in which one partner is HIV positive and the other is HIV negative.)
In a special HIV/AIDS issue of the British medical journal The Lancet, Australian researchers led by Dr David Wilson and Professor David Cooper from the University of New South Wales's National Centre in HIV Epidemiology and Clinical Research (NCHECR) have challenged the conclusions of the Swiss experts and warned infection could potentially quadruple in certain populations if HIV-positive people followed the Swiss advice.
Wilson's team used mathematical modelling to analyse the implications of the Swiss statement, finding that over a prolonged period, particularly among serodiscordant homosexual partners, the cumulative possibility of transmission is as high as four-fold.
The researchers accept that the risk of transmission in heterosexual partnerships in the presence of effective treatment is very low, but warn it is not zero. They also note that the transmission risk in male homosexual partnerships, over a prolonged period of repeated exposures, is much higher.
"While it is true that the individual risk of HIV transmission per act is fairly small for people on retrovirals, the risk of transmission over large numbers of acts could be substantial," Wilson said.
Dr Jonathan Anderson, president of the Australasian Society for HIV Medicine, who is also based at the NCHECR, said that while the viral load goes down in the blood due to antiretrovirals, that may not be the case in semen or in vaginal and anal fluids.
"This may be confusing," Anderson said. "Antiretrovirals can complement consistent condom use but replacing condom use with medications may end in disaster."
The researchers write that if the Swiss claim of non-infectiousness in effectively treated patients was widely accepted, and condom use subsequently declined, there was the potential for substantial increases in HIV incidence.