If larger tests show that the drugs work, they could be given to people at highest risk of HIV, from gay men in American cities to women in Africa.
The drugs are tenofovir (Viread) and emtricitabine, or FTC (Emtriva), sold in combination as Truvada by Gilead Sciences Inc., a California company best known for inventing Tamiflu, a drug showing promise against bird flu.
Unlike vaccines, which work through the immune system -- the very thing HIV destroys -- AIDS drugs simply keep the virus from reproducing.
Taking them daily or weekly before exposure to the virus may keep it from taking hold, just as taking malaria drugs in advance can prevent that disease when someone is bitten by an infected mosquito, scientists believe.
Monkeys suggest they are right. Six macaques were given the drugs and then challenged with a deadly combination of monkey and human AIDS viruses.
Despite 14 weekly blasts of the virus, none of the monkeys became infected. All but one of another group of monkeys that didn't get the drugs did.
''Seeing complete protection is very promising," and something never before achieved in HIV studies, said Walid Heneine, a CDC scientist working on the study.
What happened next, when scientists quit giving the drugs, was equally exciting.
''We wanted to see, was the drug holding the virus down so we didn't detect it" or was it truly preventing infection, said Folks, head of the CDC's HIV research lab. It turned out to be the latter. ''We're now four months following the animals with no drug, no virus. They're uninfected and healthy."
Years of monkey studies using tenofovir alone had shown partial protection. The scientists added the second drug, FTC, when Gilead's combination pill, Truvada, came on the market last year.
The results, announced at a scientific meeting last month in Denver, so electrified the field that private and government funders alike have been looking at ways to expand human testing.
