The monkeys who got the fake treatment were readily infected "but the animals that received the long-acting drug remained protected," said study leader Gerardo Garcia-Lerma of the CDC.
The results mirror what was seen in the CDC's early research in monkeys on Truvada, the pill that's available for HIV prevention now.
In the second study, Chasity Andrews and others at the Aaron Diamond AIDS Research Center at Rockefeller University in New York gave eight monkeys two shots of the drug, four weeks apart, and dummy shots to eight others. The animals were exposed to the virus weekly for eight weeks. Again, all animals given the fake treatment were quickly infected and those on the drug were all protected.
To see how long a single shot would last, they did a second study. The single shot protected 12 monkeys for about 10 weeks on average.
The dose used in a single shot corresponded to what people would get from a shot every three months, researchers said.
"This is really promising," said Dr. Judith Currier, an infectious disease specialist at the University of California, Los Angeles. The research "supports moving this forward" into human testing, she said.
Currier is on the program committee for the meeting in Boston where the studies were presented — the Conference on Retroviruses and Opportunistic Infections. The New York study also was published online by the journal Science.
Grant said the long-acting drug is chemically similar to certain AIDS medicines sold now that are "extremely safe, well tolerated and extremely potent." A mid-stage trial testing the long-acting shots in people as a treatment, not a prevention, is already underway, he said.
Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP