An HIV-positive person who takes anti-retroviral drugs after diagnosis, rather than when their health declines, can cut the risk of spreading the virus to uninfected partners by 96%, according to a study.
The United States National Institutes of Health sampled 1,763 couples in which one partner was infected by HIV.
It was abandoned four years early as the trial was so successful.
The World Health Organization said it was a “crucial development”.
The study began in 2005 at 13 sites across across Africa, Asia and the Americas.
HIV-positive patients were split into two groups. In one, individuals were immediately given a course of anti-retroviral drugs.
The other group only received the treatment when their white blood cell count fell.
Both were given counselling on safe sex practices, free condoms and treatment for sexually transmitted infections.
Among those immediately starting anti-retroviral therapy there was only one case of transmission between partners.
In the other group there were 27 HIV transmissions.