A simple injection to protect against AIDS is no closer today than it was when the virus that causes it was first identified in the early 1980s, one of the world’s leading HIV experts said today.

Professor David Baltimore, a Nobel laureate who is President of the American Association for the Advancement of Science, told its annual conference in Boston that the failure of every promising approach to an HIV vaccine had led many scientists to wonder whether it would ever be possible to create one.

While both public and private sector researchers have done their best, HIV’s unparalleled ability to evade the body’s immune system has defeated current medical science, Professor Baltimore said in his presidential address to the conference.

No research projects currently underway, including his own, have any realistic prospect of producing a vaccine for at least a decade, and success may take even longer, he said. “The community is depressed because we see no hopeful route to success.”

Professor Baltimore’s comments are particularly significant as he is one of the foremost authorities on the HIV virus. He won his Nobel Prize in 1975 for the discovery of reverse transcriptase, a chemical enzyme that was later found to be used by HIV to reproduce in human cells.

The lack of progress suggests that for the foreseeable future, a vaccine will not be capable of playing a role in containing the global epidemic of HIV/Aids, which affects an estimated 33 million people and kills between 2.4 million and 3.3 million each year, chiefly in sub-Saharan Africa.

“It’s such a sad topic,” he said of the search for an Aids vaccine. “We have been trying to make an HIV vaccine since the day HIV was discovered. In 1984, we were told that as the virus had been found, a vaccine should be just around the corner. History was on our side - we have been able to make vaccines versus almost all the viruses that affect humans. But we are no closer to a vaccine now than we were then.

“Every year since then, we have been saying it is at least 10 years away. I still think it is at least 10 years away. And if it has been 10 years away for 20 years, you might ask does that really mean it will never happen? I’m not prepared to say that, because I don’t want to take a pessimistic stand. This is too important to give up on.

“Our lack of success may be understandable, but it is not acceptable.”

He said there was now a consensus that approaches based on stimulating antibody production - the traditional way in which vaccines work - would not be effective against HIV because of its extraordinary ability to “cloak” itself by changing its protein shell.

The general mood of pessimism was heightened last year by the failure of clinical trials of a candidate vaccine taking a different approach, made by the drug company Merck, which many researchers had considered promising.

While Professor Baltimore has a major grant from the Bill and Melinda Gates Foundation to research a new approach to HIV therapy and treatment, he said it was a last-ditch effort with only a slim hope of success. He compared it to a “Hail Mary” play in American football - a desperate attempt at a long pass, in which the quarterback throws the ball up and prays it will be caught by a teammate.

“This is a Hail Mary,” Professor Baltimore said. “I’m not going to pretend that we have found a route.”

His approach to the problem involves using gene therapy to modify blood stem cells, so that they produce altered immune system cells that can make specialised proteins not found in nature that can attack HIV.

If this works, it will first be deployed in new HIV therapies, before his team seeks to adapt it into a vaccine.

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