French researchers reported on Sunday that an AIDS vaccine designed to treat the disease, rather than prevent it, has scored an initial success by suppressing the virus for up to a year among a small group of patients who tried it.

Although the technique is cumbersome and costly, the experiment published in an online version of the British journal Nature Medicine is being touted as ‘‘the first demonstration of an efficient therapeutic vaccine against AIDS’’.

The vaccine was tested in Brazil on 18 volunteers who were already infected with HIV, the virus that causes AIDS, but who were not yet taking any antiviral drugs. After four months, the level of HIV in their bloodstreams had been reduced an average of 80 per cent. By the end of one year, eight patients in the group had maintained a 90 per cent reduction in virus particles in their bloodstream. Four of those patients had virus levels so low that they were comparable to so-called ‘‘long-term non-progressors,’’ a rare cohort of people infected with HIV who never seem to get sick.

Unlike a conventional vaccine, this one cannot block infection from occurring. However, if the French technique could be perfected, it has the potential to keep some HIV-infected patients healthy without their having to take the three-drug ‘‘cocktails’’ of toxic antivirals. Instead, a series of injections, perhaps once a year, would keep their chronic infections in check.

The lead investigators in the French study are Dr Jean-Marie Andrieu and Dr Wei Lu of the Institute of Research for Vaccines and Immunotherapies for Cancer and AIDS, in Paris.

In an interview, Andrieu estimated that the cost of the annual therapy could be $4,000 to $8,000, less than a year’s course of antiviral drugs. He said the only side effect of the therapy was a swelling of the lymph nodes, which caused no pain. The swelling was, in fact, an indicator that the vaccine was marshalling the body’s immune system properly to ward off the AIDS virus.

Andrieu said future research will attempt to understand ‘‘why it works in some people, and not in others’’.

‘‘This is just a preliminary study, but it is encouraging,’’ said virologist Dr Jay Levy of the University of California-San Francisco AIDS Research Institute.

A critically important step would be to determine if the vaccine also reduced the amount of virus in sexual fluids. If it did so, a population of HIV infected individuals treated with the therapeutic vaccine would be less likely to transmit the virus to others.