WASHINGTON, Nov 13 (Reuters) - Thousands of people who volunteered to test an experimental AIDS vaccine that may have actually raised the risk of infection will be told if they got the actual shot, researchers said on Tuesday.

Merck & Co. Inc. <MRK.N> and academic researchers said they would "unblind" the study, meaning everyone would find out who got the active shot and who got a dummy injection.

Two international trials of the experimental vaccine were stopped in September after it became clear the vaccine did not prevent infection with the AIDS virus.

The trials were conducted in the United States, Peru, Brazil, Dominican Republic, Haiti, Jamaica, Australia and South Africa.

Earlier this month, researchers said they saw some worrying indications that the vaccine somehow raised the risk of infection, although they stressed the vaccine could not itself cause HIV infection.

They had already begun counseling volunteers that they could be at higher risk of HIV, a fatal and incurable virus that causes AIDS.

"All study volunteers will be encouraged to continue to return to their study sites on a regular basis for ongoing risk reduction counseling and study-related tests," the researchers said in a statement.

"Study investigators are being advised this week to provide this information to the volunteers; volunteers will receive additional information about the unblinding process directly from study sites."

To test vaccines and new drugs, researchers always aim for what are called placebo-controlled, double-blinded trials. These mean that neither the researchers nor the volunteers know who gets a placebo and who gets an active ingredient, so that there is no bias in determining how well the treatment works. (Reporting by Maggie Fox; editing by Will Dunham and Eric Beech)

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