In another promising development in AIDS prevention, scientists have shown that monkeys can be protected against infection with a vaginal gel even when it is used as long as three hours after sex.

If it works in humans, such a gel would be particularly useful in countries where women have little protection against domestic violence or rape, because they could apply it surreptitiously after a partner fell asleep or a clinic could administer it after a rape.

But if the technique does move into human trials, scientists said, it is more likely that women will be asked to try to use it both before and after sex.

The study, published on Wednesday in the journal Science Translational Medicine, did not show 100 percent protection. One of six macaques became infected despite the gel; all that got a placebo gel became infected.