A once-promising vaccine for AIDS may have inadvertently increased the infection risk of people participating in clinical trials, researchers said on Wednesday.
The multinational trials involving more than 3,000 HIV-negative volunteers were canceled last month after a large-scale study found it was not effective at preventing infection.
Further analysis showed that those who received the vaccine had a higher rate of infection than those who received a placebo, said US pharmaceutical giant Merck, which helped develop the vaccine.
The study volunteers who received the vaccine are being advised of their potentially increased susceptibility, Merck said.
"We are analyzing the data to try to determine if the results are due to immune responses induced by the vaccine, differences in study populations, or some other biological phenomenon we don't yet understand, or simply due to chance," said Keith Gottesdiener, vice president of Merck's vaccine and infectious disease clinical research.
"It will take some time before we understand why the vaccine did not work and why there was a trend toward more cases of infection in volunteers who received the vaccine," he said in a statement.
The experimental vaccine cannot cause infection, Merck said.
It was a modified cold virus used to deliver three synthetically produced HIV genes in the hopes of stimulating a response from the immune system.
Unlike earlier failed vaccines which tried to get the immune system to produce antibodies, the V520 vaccine stimulated T cells, the main disease fighters of the body.
These are the cells that HIV infects and uses to replicate itself, leading to a drop in the number of T cells available to fight off other infections.
It's possible that the volunteers became more vulnerable to HIV infection because the vaccine stimulates an increase in the production of T cells, a spokeswoman said.
The randomized, double-blind trials were conducted in various sites in the US, Canada, Peru, Brazil, Dominican Republic, Haiti, Puerto Rico, Jamaica, Australia and South Africa beginning in 2004.
Volunteers, who were already at high-risk of contracting AIDS, were given prevention counseling in addition to the vaccine or placebo. But dozens became infected anyway.
All but one of the infections among those given the vaccine were in male volunteers and the bulk of those infected were homosexual men.
Those with a higher level of pre-existing immunity to the modified cold virus used to deliver the vaccine were twice as likely to have been infected if they received the vaccine.
The initial analysis found 21 cases of HIV infection among the 392 men who received the vaccine while only nine cases were reported among the 386 men with a high level of pre-existing immunity who were given a placebo.
The results are "both disappointing and puzzling," said Anthony Fauci, the director of the US National Institute of Allergy and Infectious Diseases, which co-sponsored the trials.
"Certainly, the failure of this HIV vaccine product was unexpected," he said in a statement.
"But this setback should not and can not diminish our commitment to developing an effective HIV vaccine."
About 12,000 people become infected with HIV every day and vaccines have historically been the most effective tool against infections diseases like polio and smallpox.
While scientists work on developing a vaccine, politicians need to implement proven prevention methods, Fauci said.
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