A single shot of antiretroviral drugs protected lab monkeys from the AIDS virus for weeks, according to two US trials released on Tuesday that open the way to tests on humans.

In separate work, two teams of virologists found that monkeys which received a monthly injection of a prototype drug were completely shielded from the simian equivalent of HIV.

The research builds on previous trials showing that people who take a small daily doses of antiretroviral drugs can slash their risk of being infected by an HIV-positive partner by more than 90 percent.

But this protection was far lower when participants failed to take the drugs each day, highlighting the need for a monthly or quarterly injection to avert the problem.

In one of the studies, conducted at the US Centers for Disease Control and Prevention (CDC), researchers gave a monthly shot of an experimental, delayed-effect antiretroviral drug called GSK744 to six female monkeys.

Twice a week they inserted into their vagina a liquid containing a human-simian immunodeficiency virus to simulate their having had intercourse with an infected male.

None of the females treated with GSK744 became infected, but six from a control group that were treated with a placebo all became infected quickly.

The other researchers, from the Aaron Diamond AIDS Research Center at the Rockefeller University in New York, tested the same drug on 15 monkeys, but this time exposing them to the risk of anal infection. The results were identical -- none of the treated animals was infected, but all those on placebo were infected.

A first clinical trial with 175 people is expected to begin later this year in the United States, Brazil, South Africa and Malawi with GSK744, which was developed by the British pharmaceutical firm GlaxoSmithKline.

The data was presented at the Conference on Retroviruses and Opportunistic Infections, in Boston.