Scientists are developing a live vaccine for HIV that cannot infect the people it is supposed to protect.

New Scientist magazine reports that a US team has created a hybrid of HIV and another virus that can enter cells, but cannot reproduce itself once there.

The vaccine contains four HIV genes surrounded by the coat of the vesicular stomatitis virus.

The scientists, from the Center for Aids Research at the University of California in San Diego, believe it will encourage the immune system to seek out and destroy cells harbouring HIV.

They plan to test their theory in experiments on animals.

The treatment relies on priming dendritic cells - the cells which activate the immune system to attack invaders.

Destroy invaders

Dendritic cells display protein fragments of their "prey" on the surface, and this trains other components of the immune system such as T cells to recognise and destroy the same invaders or diseased cells.

The researchers' plan is to extract dendritic cells from the patient's blood and infect them with the gutted HIV.

The patient would then be injected with the treated cells to encourage their immune system to destroy HIV-infected cells.

Using real HIV, even in a severely weakened form, would be far too risky.

So the researchers have removed the genes HIV needs to replicate itself, and added genetic material from the other virus.

Previous attempts to develop an emasculated version of HIV have ended in failure when the virus recovered its capacity to cause disease.

June Kan-Mitchell, of the Karmanos Cancer Institute in Detroit who has collaborated with the San Diego team, said the new hybrid was "one step away from the virus itself".

Initial results of laboratory tests, due to appear in Blood next month, suggest the vaccine works.

Ruth Ruprecht of Harvard Medical School: "It looks like an interesting approach and has potential."

A spokesman for the HIV charity The Terrence Higgins Trust said: "Vaccine research is a vital part of the fight against the worldwide spread of HIV, and any developments in this field are welcome.

"However, there are several different strains of the virus and it remains to be seen if a single vaccine will be effective in fighting all variants of HIV."