South African researchers said on Friday they were encouraged by results from two HIV studies indicating that vaccines might one day be effective in controlling viral levels and even preventing infection.

Preliminary data from a clinical trial involving 480 uninfected people, half of them in South Africa, found that the majority of participants experienced a positive immune response after being given the HVTN 204 vaccine.

A healthy immune system can help ward off HIV infection, while those with compromised immunity are often more susceptible to contracting the virus.

"This is really good news. This is an important milestone, but we still have a long way to go," Gavin Churchyard, the study's principal investigator, said at a briefing sponsored by the South African AIDS Vaccine Initiative in Johannesburg.

Churchyard, who heads South Africa's Aurum Institute for Health Research, added that the vaccine's side-effects generally had been mild to moderate for those involved in the trial.

A separate clinical trial of a DNA vaccine developed by Finland's FIT Biotech showed equally promising early results in a smaller HIV-positive group, most of which was made up of residents from Soweto, the black township south of Johannesburg.

Some 5.5 million South Africans, or about 12 percent of the total population, are infected with HIV, and 1,000 die each day from AIDS, making the country a frontline for HIV vaccine research.

Dr. Eftyhia Vardas, the principal investigator for the Finnish vaccine trial in South Africa, said the vaccine helped patients maintain low viral loads and steady CD4 cell counts, both key to keeping HIV from progressing into AIDS.

The studies provided a bit of blue sky in a field of AIDS research that has generally been a huge disappointment. Previous vaccine trials that have reached the critical third phase, when effectiveness is measured, have all failed.

While some AIDS scientists have turned their attention to other potential preventative weapons - microbicidal creams and mass male circumcision for example - others continue to pursue the vaccine route, though admittedly with lowered expectations.

Churchyard said that vaccines, if successfully developed, would more likely be of therapeutic rather than preventative value, taking their place alongside the anti-retrovirals that have become the cornerstone of current HIV treatment.