The announcement Friday from US pharmaceutical giant Merck that it was halting a clinical test of the HIV vaccine was greeted with dismay after high hopes had accompanied the introduction of the V520 vaccine.
"It is a huge disappointment because this vaccine has shown promise all the way through, but it's only when you get in on these big trials that you start to see how the vaccine behaves," Glenda Gray, one of the principal investigators for the clinical trial, told AFP.
"Although in earlier studies we saw beautiful immune responses, it doesn't look like this immune response translates into something that could protect people against HIV infection," Gray added from South Africa, which is home to nearly 5.5 million people with HIV, which causes AIDS.
Merck and a center funded by the US government called off further trials of the vaccine after a large-scale study found it failed to prevent infection or to reduce the amount of the virus in the bloodstream.
The trial launched in 2004 used 3,000 volunteers in the United States and Latin American countries from between 18 and 45 years of age who were at high risk of infection of HIV.
Unlike traditional vaccines that proved ineffective against AIDS, which focused on causing the immune system to produce antibodies, the V520 offered a new approach through stimulating T cells, a component of the immune system.
Earlier experiments on animals and smaller-scale tests on humans had provided encouraging results, prompting Merck and US researchers to develop the vaccine using a common cold virus to deliver three genes from the AIDS virus.
But the new strategy failed to deliver in the larger clinical trial.
"We share in the disappointment of the research and HIV communities today," said Peter Kim, president of Merck Research Laboratories, in a statement Friday.
"Sadly, developing an effective AIDS vaccine remains one of the most challenging tasks facing modern medicine."
Merck and the US researchers who oversaw the study promised to share details of the trial with other scientists.
Larry Corey, principal investigator of the HIV vaccine trials network involved in the study, said "the data from this trial will provide critical insights into this disease and future vaccine development."
While the results were disappointing, it was too early to draw conclusions about the new class of vaccine, Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases which co-sponsored the trial, told the New York Times.
The failure of the V520 vaccine illustrated how AIDS is different from other infections which scientists have developed vaccines for in the past.
The daunting search to find an anti-AIDS vaccine has involved dozens of clinical trials in recent years with tests for some 30 vaccine candidates still under way.
In May 1997, then US president Bill Clinton declared securing a vaccine in a decade a national priority, but the latest clinical tests have underlined the difficulty facing scientists.
"Creating a vaccine to prevent transmission of HIV, or at least to limit its pathogenic and epidemic potential, is one of the great challenges of our time," Bette Korber, a researcher at Los Alamos National Laboratory in New Mexico, said at a conference last year.
Since the identification of the HIV virus in 1981, the number of those infected has continued to rise with some 40 million carrying the virus. Each year brings an estimated four million new infections, of which 90 percent occur in developing countries.
Over the last 25 years, AIDS has claimed more than 25 million lives, with a majority of victims in sub-Saharan Africa.