Using HIV treatment drugs to reduce the risk of spreading the AIDS virus may be even more effective than thought, according to new analysis from a landmark trial presented here on Monday.

The four-day meeting in Rome under the International AIDS Society was given the first full peer-reviewed data from a trial whose preliminary results were unveiled to media in May to astonishment and acclaim.

These figures found that giving an HIV-infected patient early treatment reduced the risk of transmitting the human immunodeficiency virus (HIV) by sex to a non-infected partner by 96 percent.

That performance puts early therapy on a par with a condom, a prevention often shunned by people at risk.

Monday's analysis suggested this already-stellar protectiveness could even be a shade higher.

"(It) may be even stronger than initially reported," the investigators said in a press release.

Named HPTN 052, for HIV Prevention Trials Network 052, the project entailed enrolling 1,763 "serodiscordant" couples, meaning couples where one partner was infected by HIV while the other was HIV-free.

It took place at 13 sites in Botswana, Brazil, India, Kenya, Malawi, South Africa, Thailand, the United States and Zimbabwe.

In one group, the HIV-infected partners began taking HIV drugs immediately.

In the other, the infected partner delayed taking the therapy until his or her count of immune cells or state of health met guidelines for initiating treatment under the UN's World Health Organisation (WHO).

Initial data found that 39 people became infected in the course of the study.

A triple test to determine the source of this infection found that 27 cases occurred from infected partners in the delayed treatment group, and one in the immediate treatment group.

The 11 other cases were caused by sex with non-partners or were undetermined at that point.

The latest data found 28 cases among the delayed treatment group -- one more than before.

It also discovered that the sole case in the immediate treatment group was a person who probably became infected close to the time when the couple enrolled in the study and before HIV drugs could suppress the virus in body fluids.

"The protection is going to be greater than 96 percent," Myron Cohen, the trial's lead investigator and professor of microbiology and immunology at the University of North Carolina at Chapel Hill, told AFP.

In a further benefit, early use of drugs was also associated with a 41-percent reduction in sickness or death related to HIV.

When these figures are put together, it means that the benefits of immediately starting HIV treatment are even stronger than thought, said the study.

The WHO hailed "the exciting results" of the study and said they would be factored into guidelines for using antiretrovirals and testing and counselling couples.

When investigators began enrolling volunteers in April 2005, the UN's health agency recommended initiating the drugs for anyone with advanced HIV disease or with a count of CD4 immune cells that were less than 200 per microlitre of blood.

For the delayed treatment group, the study used a benchmark of 250, and the WHO itself revised its guidelines last year to 350 CD4 cells per microlitre of blood.

The Rome conference gathers 5,500 specialists, ranging from virologists to pharmacologists and disease trackers.

It is staged once every two years by the International AIDS Society (IAS), which also organises the International AIDS Conference, a bigger event that touches on the pandemic's many social dimensions.

The study was also published concurrently by the New England Journal of Medicine.

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