Most scientists involved in Aids research believe that a vaccine against HIV is further away than ever and some have admitted that effective immunisation against the virus may never be possible, according to an unprecedented poll conducted by The Independent.

A mood of deep pessimism has spread among the international community of Aids scientists after the failure of a trial of a promising vaccine at the end of last year. It just was the latest in a series of setbacks in the 25-year struggle to develop an HIV vaccine.

The Independent's survey of more than 35 leading Aids scientists in Britain and the United States found that just two were now more optimistic about the prospects for an HIV vaccine than they were a year ago; only four said they were more optimistic now than they were five years ago.

Download the new Independent Premium app Sharing the full story, not just the headlines

Nearly two thirds believed that an HIV vaccine will not be developed within the next 10 years and some of them said that it may take at least 20 more years of research before a vaccine can be used to protect people either from infection or the onset of Aids.

A substantial minority of the scientists admitted that an HIV vaccine may never be developed, and even those who believe that one could appear within the next 10 years added caveats saying that such a vaccine would be unlikely to work as a truly effective prophylactic against infection by the virus.

One of the major conclusions to emerge from the failed clinical trial of the most promising prototype vaccine, manufactured by the drug company Merck, was that an important animal model used for more than a decade, testing HIV vaccines on monkeys before they are used on humans, does not in fact work.

This has meant that prototype HIV vaccines which appear to work well when tested on monkeys infected with an artificial virus do not work when tested on human volunteers at risk of HIV – a finding that will be exploited by anti-vivisectionist campaigners opposed to vaccine experiments on primates.

Anthony Fauci, the director of the US National Institute of Allergy and Infectious Diseases (NIAID), near Washington, told The Independent that the animal model – which uses genetically engineered simian and human immunodeficiency viruses in a combination, known as SHIV – failed to predict what will happen when a prototype vaccine is moved from laboratory monkeys to people. "We've learnt a few important things [from the clinical trial]. We've learnt that one of the animal models, the SHIV model, really doesn't predict very well at all," he said.

"At least we now know that you can get a situation where it looks like you are protecting against SHIV and you're not protecting at all in the human model – that's important," he said.

The NIAID spends about $500m (£250m) on HIV vaccine research each year and despite calls from some Aids pressure groups for funds to be diverted to other forms of Aids prevention, Dr Fauci said this was not the time to stop vaccine research. "I don't think you should say that this is the point where we're going to give up on developing a vaccine. I think you continue given that there are so many unanswered questions to answer," he said. "There is an impression given by some that if you do vaccine research you are neglecting other areas of prevention. That's not the case. We should and we are doing them simultaneously."

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

More than 80 per cent of the scientists who took part in our survey agreed that it was now important to change the direction of HIV vaccine research, given the failure of the Merck clinical trial, which was cancelled when it emerged that the vaccine may have actually increased the chances of people developing Aids.

Robert Gallo, a prominent Aids researcher in the US who is credited with co-discovering the virus in the early 1980s, likened the vaccine's failure to the Challenger disaster, which forced Nasa to ground the space shuttle fleet for years.

At the end of last month, Dr Fauci convened a high-level summit of leading HIV specialists at a hotel in Bethesda, Maryland, to discuss the future direction of research. A group of 14 prominent Aids specialists had already written to Dr Fauci suggesting that his institute had "lost its way" in terms of an HIV vaccine.

He said that one outcome of the meeting was a refocusing of the vaccine effort away from expensive clinical trials towards more fundamental research to understand the basic biology of the virus and its effects on the human immune system.

"We'll be turning the knob more towards answering some fundamental questions rather than going into big clinical trials," Dr Fauci said. "I'm certainly disappointed that we're not further ahead in the development of a vaccine but I don't say that this year I'm more discouraged than I was last year. I always knew from the beginning that it would be a very difficult task given what we know about this very elusive virus."

About 33 million people in the world are infected with HIV and some 26 million have died of Aids since the pandemic began.

The majority of scientists who responded to The Independent's survey said that a vaccine would be the most effective way of preventing the spread of the virus given the failure of many education programmes.

Winnie Sseruma, 46: 'For me, the key has been not to give up'

Ms Sseruma says she believes abandoning research for a vaccine would mean a loss of hope for millions of people. "When I was diagnosed, nearly 20 years ago, it was when the first drugs had come on the market. A lot of people had said before then that there was no hope and that all efforts should be put into prevention. But look where we are now. We cannot lose hope; we need to invest in a vaccine."

She says this latest failure needs to be seen as the first hurdle, not a signal to give up. "Yes, the scientists have not been very successful in their quest for a vaccine, but you can learn a lot from failures. Now they have realised they cannot use the normal routes used to develope simpler vaccines."

Ms Sseruma lives in London, but was born in Uganda and says that the current climate of pessimism for the vaccine is not dissimilar to the initial doubts over the likelihood of treating HIV in Africa.

"I remember when treatment started being available in the West and people were saying it would be impossible to send it to Africa. But look what's happened. We should always do whatever is humanly possible to fight Aids. It's been a long journey, but for me, the key has been not to give up, and the scientists need to have the same attitude."

'Philippe B', 42: 'People are getting resistant to drugs'

"Philippe", who wishes to remain anonymous, discovered he was HIV positive 11 years ago. The 42-year-old believes the search for the vaccination should no longer be a priority, but that it should not stop altogether.

"Unfortunately what's happening now is that people are getting more resistant to drug treatment, and more money needs to be put into finding more drugs for treatment," he said.

For people like Philippe, the fear of building an immunity to drugs and running out of options is a real one. He believes that as long as scientists are still pessimistic about the chances of successfully finding a vaccine, money needs to be invested in continuing to fund research into treatment.

"I've already become resistant to five combination treatments over the last ten years, and if I was on the last one available I'd be very afraid. HIV is not a death sentence in the way it once was, but we do need to fund further research into the drugs that treat it."

Nevertheless, Philippe thinks it is not yet time to abandon all research into a vaccine. "In my lifetime I don't think we'll have a vaccine, but there's no reason we should believe it isn't possible," he said. "But we should now be spending more on other ways of dealing with the disease."

Click here to have your say