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As the search for an effective HIV prevention strategy intensifies, scientists are hoping that antiretroviral (ARV) drugs, normally associated with HIV treatment, may provide part of the answer.

"We are in a desperate race against time in pursuit of prevention that works," former UN Special Envoy for AIDS in Africa Stephen Lewis told journalists at the International AIDS Conference in Mexico City on Tuesday.

Lewis, who is currently co-director of AIDS-Free World, an international advocacy organisation, remarked that during his tenure as UN Special Envoy, he had spent almost five years "begging" governments in Africa to roll out treatment to those who needed it. "If I had been able to say ... 'not only will [ARVs] keep people alive, but they can significantly reduce new infections', this would have been a huge inducement."

A study by Canada's British Columbia Centre for Excellence in HIV/AIDS recently found that providing ARV treatment to HIV-positive people could lower the number of new HIV infections by as much as 60 percent.

The theory is that higher concentrations of the HI virus in the body (viral load), increase the likelihood of transmission. ARV therapy reduces the viral load in the blood, as well as in genital secretions in both men and women, making HIV-infected people potentially less contagious.

Several studies have shown that in discordant heterosexual couples (where one partner is positive and the other negative) the odds of the negative partner becoming infected are very low when the positive person's viral load has dropped significantly as a result of treatment.

The Canadian researchers used a new mathematical model to find out whether providing treatment to more people living with HIV in British Columbia would reduce future cases in the province.

The results showed that offering the life-prolonging medication to 75 percent of HIV-positive people would reduce the annual number of HIV cases in British Columbia by 30 percent; if 100 percent of HIV-positive people received the drugs, the number of new cases would drop by 60 percent.

"We've known for some time that the expansion of coverage of highly active ARV therapy could help to reduce the number of new infections ... we were amazed at the actual number of new infections that can be potentially averted by expanding access to treatment," said Dr Julio Montaner, head of the British Columbia Centre for Excellence in HIV/AIDS.

Montaner, who led the study, called for more research, but warned that treatment alone would not be enough to prevent new HIV infections; a combination of prevention interventions was the only solution.

During his Tuesday plenary presentation, Myron Cohen, a researcher at the University of North Carolina, agreed that there were still a number of unanswered questions about using ARVs for prevention, and that it would be impossible to "treat our way out of the epidemic". Nevertheless, he told delegates that the time had come to "marry" HIV prevention and treatment.

Cohen is leading a clinical trial of 1,750 discordant couples in six countries to determine the effects of starting treatment early on the transmission of HIV to uninfected partners. Results of the trial, which is being conducted by the HIV Prevention Trials Network of the National Institute of Allergy and Infectious Diseases, will probably only be available in five or more years' time.

A pill a day keeps the virus away?

In the meantime, trials looking into the efficacy of pre-exposure prophylaxis (PrEP), in which ARVs are regularly given to HIV-negative people who are at high risk of infection, are either planned or underway in countries in Africa, Asia, Latin America and North America.

According to the AIDS Vaccine Advocacy Coalition (AVAC), more people will be enrolled in PrEP trials than in all of those for vaccines and microbicides combined.

"Although still unproven by human clinical research, PrEP is considered one of the promising clinical interventions against HIV currently in development," said Mitchell Warren, executive director of AVAC, earlier this week.

AVAC released a new report at the conference, calling for increased action by governments, donors, researchers and advocates to prepare for initial results from the first PrEP trials, which are expected as early as 2009.

"We should look ahead to consider all of the possible outcomes of these trials, and make real plans for making PrEP available to those who can benefit from it as quickly and safely as possible if it is proven effective," urged Pedro Goicochea, an investigator for a PrEP study underway in Peru and Ecuador.