BOSTON — Researchers are reporting that injections of long-lasting AIDS drugs protected monkeys for weeks against infection, a finding that could lead to a major breakthrough in preventing the disease in humans.

Two studies by different laboratory groups each found 100 percent protection in monkeys that got monthly injections of antiretroviral drugs, and there was evidence that a single shot every three months might work just as well.

If the findings can be replicated in humans, they have the potential to overcome a major problem in AIDS prevention: that many people fail to take their antiretroviral pills regularly.

A preliminary human trial is to start late this year, said Dr. Wafaa El-Sadr, an AIDS expert at Columbia University’s Mailman School of Public Health, but a larger trial that could lead to a treatment in humans may still be some years away.