Drugs that repress HIV halved the number of new cases of the AIDS virus, according to a study published today ahead of the opening of the International AIDS Conference.
The findings support those who argue that antiretrovirals, which treat the human immunodeficiency virus (HIV) but do not eliminate it completely, are also a potent weapon for preventing viral spread.
The paper looked at the population of the Canadian province of British Columbia, examining coverage of antiretroviral treatment and new cases of infection between 1996 and 2009.
Over this period, the annual tally of new cases fell by 52 percent, the researchers found.
For every 100 patients that were placed on the drugs, new diagnoses of HIV fell by three per cent.
The paper, published in The Lancet, is authored by Julio Montaner, director of the British Columbia Centre for Excellence in HIV/AIDS in Vancouver, and president of the International AIDS Society.
The time scale of the study dates back to the introduction in 1996 of the triple HIV drug cocktail, which now provides a lifeline for five million badly-infected people.
During the period under study, the number of people on this combination therapy rose from 837 to 5,413 but the number of new HIV diagnoses fell from 702 to 338 per year.
Concentrations of virus in the blood among treated patients also fell sharply.
Antiretrovirals can reduce levels of HIV to below detectable levels. The virus retreats to "reservoirs" such as the lymph glands, where so far no way has been found to eliminate it and if the drugs are stopped, it rebounds.
But having very low levels of virus also logically implies a fall in the risk of handing on the pathogen to others.
Previous studies have also highlighted the indirect benefit of antiretrovirals in preventing new cases of HIV.
As a result, some specialists say the time is near to declare antiretrovirals a preventative tool, alongside the condom, that should be added to the panoply of options for preventing spread of HIV.