In what could eventually become a milestone for H.I.V. prevention, very preliminary tests of an implant containing a new drug suggest that it may protect against infection for a full year.

The new implant, by the drug company Merck, was tested in just a dozen subjects for 12 weeks. But experts were quite excited at its potential to revolutionize the long battle against H.I.V.

The research was described on Tuesday at an international AIDS conference in Mexico City.

New H.I.V. prevention methods are desperately needed. About 75 million people have contracted the lethal virus since the AIDS epidemic began. Even now, about 1.7 million people are infected each year — despite decades of promotion of condoms and abstinence, and years of efforts to get people to take a daily pill that prevents infection.

“If — and I’m emphasizing if — if it pans out in a larger trial that it delivers a level of drug that’s protective for a year, that would be a game-changer,” said Dr. Anthony S. Fauci, director of the National Institute for Allergy and Infectious Disease and a leading expert on AIDS.