The National Institute of Allergy and Infectious Diseases halted the study of the HVTN 505 vaccine after researchers found volunteers were slightly more likely to acquire human immunodeficiency virus than those who received a placebo.
Smaller trials of the genetically engineered drug had hinted at a possible breakthrough in the search for a cure for HIV, the NIAID trial of 2,500 people showed a "non-statistically significant increase in HIV acquisition among volunteers" after three years of study.
NIAID, which is part of the National institutes of Health, announced the decision in a statement Thursday.
Researchers hoped the vaccine would teach the immune system to recognize the genetic markers of the virus and fight against it.
While the slight increase could be a matter of chance, the data showed the vaccine was unlikely to ever defend against the virus. It also failed to decrease the viral load -- the measure of the infection's severity -- in those who contracted HIV during the trial.
"This is quite a substantial disappointment," Dr. Scott Hammer, a virologist at Columbia University who was one of the study's leaders, told the Los Angeles Times. Still, he said, "we've learned from every clinical efficacy trial we've done. We've had good and bad news, but each one takes us a little closer in terms of what to pursue and not to pursue."
The HVTN 505 vaccine was developed improving on a previous vaccine, Aidsvax, an early trial of which called RV-144 showed a 30 percent reduced infection among Thai heterosexuals.
Approximately 34 million people worldwide live with HIV, according to World Health Organization estimates, 2.5 million of whom became infected in 2011. HIV infection can lead to AIDS, a condition in which the immune system becomes ineffective and makes the person susceptible to infections easily fought by healthy individuals.
Despite the disappointing results of HVTN 505, researchers plan to continue to use the blood samples from the study's participants in the hopes that this failure will open the door to the eventual cure.