A clinical trial testing the effectiveness of what was thought to be a promising new HIV vaccine has ended in failure.

The vaccine, made by the pharmaceutical company Merck, offered no protection against infection by the virus, which can culminate in Aids (Acquired Immune Deficiency Syndrome).

Scientists have been trying in vain to create an effective vaccine against human immunodeficiency virus (HIV) for more than 20 years, but some now doubt that tests on animals will ever produce a vaccine that will be effective in humans.

Experts described the results of the US trial, published in an online edition of The Lancet medical journal, as “disappointing”.

The Step Study, led by Susan Buchbinder, from the San Francisco Department of Public Health, compared the vaccine with a non-active placebo in almost 3,000 patients.

All were free of HIV infection at the start of the trial but considered to be at “high risk” because of their lifestyles.

The study showed that the vaccine, known as MRKAd5 HIV-1 gag/pol/nef, totally failed to protect against HIV infection.

In fact some people given the vaccine had a slightly higher rate of infection than those receiving the placebo, causing the trial to be called off prematurely in July this year.

The vaccine was designed to trigger a protective response by “killer” cytotoxic T-cells, white blood cells that are a key element of the immune system.

It had previously shown positive results when tested on monkeys exposed to a simian-HIV (SHIV) viral infection, a mixture of simian and human forms of the virus.

Dr Merlin Robb, from the US Military HIV Research Program in Rockville, Maryland, said: “The Step results have profoundly affected the HIV-vaccine development field. The predictive value of the non-human SHIV-challenge model is not supported by this experience.

“The failure of Step has not closed the door on the T-cell vaccine concept. However, the reflection it has caused might be a crucial step towards an HIV vaccine.”

Lisa Power, Corporate Head of Policy at the Terrence Higgins Trust charity said: “HIV vaccine development is a long term goal with many short term setbacks.

“However, it’s always disappointing when a promising avenue of inquiry turns out to be empty.”

Infection expert Professor Robin Shattock, from St George’s, University of London, said: “While the results of this trial were disappointing, this study was still an important step in determining what is required for an HIV vaccine.

“We now know that the vaccine approach tested in this trial produced a response against too few viral targets using only one aspect of the immune response — cytotoxic T-cells.”

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