The most promising of the many experimental HIV vaccines in development will offer only limited immunity against the deadly virus, US government scientists said Wednesday ahead of world HIV Vaccine Awareness Day.

Unlike classical vaccines, the first-generation HIV vaccines will not enable people to fight off the virus, but may protect their immune system from the worst ravages of the virus and delay the onset of AIDS.

And because patients are at their most infectious when viral levels are high, such vaccines might also reduce the spread of the sexually-transmitted disease, making it a useful tool for public health authorities battling to contain the global HIV/AIDS pandemic.

"There is optimism that even a less-than-perfect vaccine could benefit both individual recipients and the at-risk community," said the authors in a commentary published in the New England Journal of Medicine.

It is still not clear when the first vaccines will be available.

However, stage I and II clinical trials are "well into their execution. Large numbers of people are being vaccinated," said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), and one of the authors of the paper.

Researchers around the globe have been searching for a vaccine against the human immunodeficiency virus (HIV) for two decades, but efforts have been thwarted by the virus' ability to mutate and evade clearance by the immune system.

Eventually scientists turned their attention to so-called T-cell vaccines that primarily induce a cellular immune response that reduces viral levels and preserves critical cells needed to control infection.

In animal studies, peak viral levels were reduced by a factor of 10 in primates that were inoculated with these types of vaccines and then infected with the simian counterpart of the HIV virus, according to the paper.

The inoculations also "dramatically" slowed the progression of the disease in many animals, Fauci said.

The hope is that an effective human T-cell vaccine could substantially improve the quality of life for people who contract the virus after immunization by postponing the day when they develop AIDS and have to begin treatment with a daily cocktail of drugs.

Also, by stifling the initial burst of virus and better controlling virus levels, an immunization program could potentially curb the spread of the epidemic, which is largely driven by people who have viral loads.

Computer modeling studies have suggested that even a vaccine that does not provide adequate protection against infection might alter the course of the epidemic, according to the paper, but further studies will be needed to test the hypothesis, the authors said.

Some 40 million people are HIV-positive and another 11,000 people contract the virus every day, most of them in the world's poorest nations, according to the NIAID.

The commentary was published to coincide with the 10th anniversary this Friday of HIV Vaccine Awareness Day.

Individuals with key variants in an important immune cell and a molecule that controls it show a slower progression to AIDS after they are infected with the human immunodeficiency virus (HIV), a study released on Sunday says.

The paper focuses on "natural killer" cells, which are unleashed by the immune system to crush viral intruders.

Natural killers are switched on or off by receptors, or docking sites, on their surface. The receptors are activated by a molecule presented to the cell by the immune system's signallers.

Researchers led by Mary Carrington of the United States' National Cancer Institute, Maryland, looked at variants in two genes -- one that creates a receptor named KIR3DL1, and one that creates a signalling molecule called HLA-B.

In a study of 1,500 people with HIV, they found that individuals who had specific variants in both genes were helped "significantly and strongly," progressing to AIDS much later than counterparts without these variants and also having lower levels of virus in the blood.

The study is released online by Nature Genetics.

Meanwhile, work published in a sister journal, Nature Immunology, casts light on how HIV can foil the immune system by stifling a sentry cell called a dendritic cell.

By latching onto the DC-SIGN receptor on this cell, the virus blocked the signalling pathway, enabling itself to swarm around nearby immune T-cells and penetrate them.

Around 39.5 million people were living with HIV or AIDS at the end of last year, according to the World Health Organisation (WHO) and UNAIDS.