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Government halts HIV vaccine trial after shot fails to prevent infections

A U.S. government study of an experimental HIV vaccine has been halted effective immediately, because the vaccine did not prevent HIV infection in some volunteers.

The study, called the "HVTN 505 clinical trial," kicked off in 2009 and had enrolled about 2,500 people in 19 cities. Participants were all men who have sex with men and transgender people who have sex with men. Half received an experimental vaccine developed by the National Institutes of Health, and half received dummy, or placebo, shots.

A scheduled safety review on April 22 found that slightly more volunteers who had received the vaccine later became infected with HIV. Overall, 41 cases of HIV infection occurred in the volunteers who received the experimental vaccine and 30 cases of HIV infection occurred among the recipients who received the dummy injection.

It's not clear why, but the National Institute of Allergy and Infectious Diseases - a branch of the NIH -- noted the increased risk of HIV among vaccine group was not statistically significant when compared to the placebo group, meaning the findings may be due to chance.

The safety review also showed the vaccine failed to reduce the amount of HIV virus in the blood, called the "viral load," in people who had been diagnosed with HIV and were tracked for 20 weeks of follow-up.

The NIH said in a statement Thursday that it is stopping vaccinations, but will continue to study the volunteers' health.

HIV Vaccine on the Horizon?

Multiple attempts at creating an AIDS vaccine have failed over the years. But researchers continue to try, pointing to modest success in a 2009 study in Thailand. (At left is a 2009 CBS Evening News report on that study)

"NIAID remains committed to the pursuit of a highly effective, preventive HIV vaccine as part of a multifaceted HIV prevention research program," the agency said in a statement.

The NIAID has more information on the failed vaccine trial in a "questions and answers" website.