Treating HIV patients early, before they are too sick, dramatically lowers their chances of spreading the AIDS virus to a sexual partner, a major study finds.

The nine-country study confirms what scientists have long believed: HIV medicines do not benefit just patients' own health but act as prevention by making those people less infectious. Earlier treatment meant patients were 96 per cent less likely to spread HIV to their uninfected partners.

Stephen Farrar shows some of the AIDS medications he takes on a daily basis, in Fort Lauderdale, Fla. Earlier treatment meant patients were 96 per cent less likely to spread HIV to their uninfected partners, researchers found. Wilfredo Lee/Associated Press

The findings were striking enough that the U.S. National Institutes of Health announced Thursday it was stopping the study four years ahead of schedule to get the word out.

Condoms still are crucial for protection. All 1,763 couples in the study, where one partner had HIV and the other did not, were urged to use them.

But the findings promise to play a role in an important decision: Antiviral drugs are life saving, but also expensive and side-effect-prone, so how early should patients start taking them? In the U.S., that's a case-by-case decision for patients whose immune systems so far are moderately damaged by HIV. In developing countries, patients tend to be sicker before treatment starts.

The study may change those guidelines by adding the promise of partner protection.

"It has less to do with a decision about what's good for you from a personal health standpoint than what is the extra added benefit from starting earlier, i.e., transmission, especially if you have a partner who's uninfected," said Dr. Anthony Fauci, director of NIH's National Institute of Allergy and Infectious Diseases, which oversaw the study.

The study randomly divided the couples. Among half, the HIV-infected partner immediately started medication. Among the other half, the infected partner delayed medication until their level of CD4 cells, a key measure of immune health, dropped below 250 or they caught other AIDS-related illnesses.

In 28 couples, the uninfected partner became infected with a strain of HIV that scientists could prove came from the originally infected partner. Only one of those infections was among the earlier-treated couples, Fauci said.

Last year, Dr. Julio Montaner of the B.C. Centre for Excellence in HIV/AIDS drew similar conclusions, showing that for every 100 HIV patients in the province who received anti-retroviral therapy, new diagnoses of the disease fell by three per cent.

Anti-retrovirals are expensive, Montaner said, but using them to prevent infection will ultimately save health-care dollars. Each new HIV infection in North America costs about $250,000 to $500,000 in a lifetime, he said.

Researchers stress that safe sex is still a crucial part of AIDS prevention.

With files from CBC's Justin Hayward