The first Aids vaccine to be tested on humans does not protect the general population as a whole. But it may protect two groups within it, black and Asian people, a US biotech company has announced.
The company, VaxGen, has completed a three-year study involving more than 5,000 volunteers.
Initial results show the vaccine only reduced the rate of HIV infection by 3.8%, VaxGen said in a statement.
Black and Asian people who were given the vaccine, however, had a 67% lower rate of infection than those who received a placebo shot.
Scientists remain confident that an Aids vaccine is possible
But these subgroups were small, which means the results have to be treated with caution.
To be granted a licence, scientists needed to show that any Aids vaccine was effective in at least a third of patients.
This is the first look at the potential for a human vaccine against the deadly virus, which killed more than three million people worldwide last year.
There is no cure for HIV, and drugs that can keep it under control are only readily available in rich countries.
Gay men and women thought to be at high risk of contracting HIV received up to seven injections during the trial. Some received the vaccine and others the placebo shot.
The 5,000 men and 400 woman volunteers have been monitored since 1998 to see if they became infected.
This vaccine, known as AidsVax, is the only HIV vaccine to have completed Phase III clinical trials - the last step before drug companies can seek approval from the US Food and Drug Administration.
It is designed to protect against two strains of the HIV virus found in the West.
There is no risk of the vaccine itself causing Aids as it contains no genetic material from the HIV virus.
Instead a synthetic copy of proteins found in HIV is used to try to sensitise the body's immune system to the virus so that it produces anti-bodies to fight it.
The hepatitis B vaccine successfully uses the same approach.
A number of other vaccines based on the same principle are in earlier stages of development.
Lance Gordon, chief executive officer of VaxGen, said: "We intend to continue development of this vaccine through
licensure, including additional studies as necessary, for use in groups in which the vaccine demonstrated a significant reduction in infection.
"In parallel, we will continue our work on the vaccine to make it more broadly effective."
Dr Peter Piot, executive director of UNAIDS, said the results were "promising".
He said: "The trial provides clear evidence that a vaccine can work.
"However, there is an urgent need for more targeted research to find out why the candidate vaccine only seems to work in certain population sub-groups.
"In the meantime, we must continue to expand existing prevention efforts, which have proved their effectiveness when they are implemented at full scale."
Dr Seth Berkley, president and of the International
Aids Vaccine Initiative, called the announcement
But he said: "We are not discouraged. The search for an Aids vaccine will and must go on.
"Scientists remain confident that an Aids vaccine is
"Alternative Aids vaccines, employing different design
strategies, are now in development, and some have already
entered human trials.
"These must move forward through further study, without delay."
Nick Partridge, chief executive of the UK charity the Terrence Higgins Trust, said the results showed just how difficult it was to develop an effective HIV vaccine.
"With over 43 million people living with HIV across the world, it is vital that other vaccine trials being conducted continue to be fully supported.
"In the meantime, good sex education and HIV prevention programmes still remain our biggest weapon against the continued spread of the virus."
Simon Wright, of the charity Action Aid, said: "In some ways it confirms what we've known for a long time which is that there isn't going to be a vaccine available in developing countries in any significant way in the foreseeable future."
The National Aids Trust called for increased funding for the drive to find an effective vaccine.