Results of a study conducted on monkeys raise hope for a drug that could prevent HIV infection in humans that involve getting a shot in the arm once every one to three months.

The human immunodeficiency virus (HIV) is responsible for the dreaded acquired immunodeficiency syndrome (AIDS) and is primarily spread through transfer of blood, vaginal fluid and semen. To date, no vaccine has yet been developed to prevent HIV and although using condoms is considered as one of the best ways to prevent infection, not everyone uses them all the time. Public health officials therefore pursue other options that could prevent the spread of HIV and other sexually transmitted diseases.

An antiretroviral drug that treats people infected with HIV and prevents infection in people who do not have the virus has already been developed by Gilead Sciences. A study involving gay men even found the drug to be effective in reducing risks by up to 90 percent.

The drug, Gilead's Truvada, however, needs to be taken daily and this poses a problem to some people who may forget or ignore the importance of taking the drug everyday. "It's hard to get healthy people to take a pill or put on a salve every day," said Robert Schooley, an infectious-disease specialist at the University of California in San Diego.

In a study involving an experiment with macaque monkeys, and reported in the March 4 issue of Science, researchers have found an option to taking daily pills. In the study, the researchers gave one group of monkeys shots of the experimental drug called GSK744 once every four weeks. Another group of monkeys did not receive the shots and served as the control group. All the monkeys were then exposed to HIV once a week for eight weeks.

The monkeys in the second group, which received the dummy shots, were readily infected but those in the first group that received the long acting antiviral drug were protected from infection.

In a follow-up experiment, the researchers observed that a single dose of the drug protected the monkeys for up to ten weeks, which researchers said translates to up to three months of protection in humans.

"With a quarterly dosing schedule in humans, our results suggest that GSK744 LA could potentially decrease adherence problems associated with daily preexposure prophylaxis (PrEP)," the researchers wrote.

The researchers also said the new drug offers a more practical option to getting protection from HIV. "We believe it's a very practical, feasible approach to HIV prevention," said study author David Ho, a virologist at the Aaron Diamond AIDS Research Center in New York.