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. and academic researchers said they would "unblind" the study, meaning everyone would find out who got the real shot and who got a placebo injection.
"All study volunteers will be encouraged to continue to return to their study sites on a regular basis for ongoing risk reduction counselling 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."
Some 3,000 people in the United States, Peru, Brazil, Dominican Republic, Haiti, Jamaica, Australia and South Africa -- mostly gay men and female sex workers -- had volunteered to test the new vaccine. All were warned to protect themselves from AIDS exposure.
The vaccine was being tested to see if it could prevent users from acquiring HIV, the virus that leads to AIDS. But two international trials were stopped in September after it became clear the vaccine did not prevent infection.
In fact, researchers said earlier this month they saw some worrying indications that the vaccine actually raised the risk of infection. They could not explain the effect but stressed the vaccine could not in itself cause infection.
None of the volunteers knew whether they had received the real virus or not. That's because the studies were randomized, double-blinded trials. "Double-blind" means that neither the researchers nor the volunteers knew who was receiving the real vaccine and who was getting the placebo, so that there would be no bias in testing how well the treatment worked.
The Merck vaccine, known as V520, was made from a common cold virus with three synthetic HIV genes tucked inside. It was hoped that it would stimulate the immune system to kill any HIV-infected cells it encountered.
During the course of the study, 49 of 914 vaccinated men became infected with HIV, compared with 33 of the 922 men who got the placebo shot, Merck reported. Only one woman and a small number of heterosexual men were infected.
It's still unclear why more of the vaccinated volunteers wound up getting HIV than those who got the placeboes. Dr. Keith Gottesdiener, vice president of clinical research at Merck Research Laboratories, said one possibility is that the vaccine could have somehow made people more susceptible to HIV infection. It's also possible that the higher numbers were simply coincidence.
Merck's head of medical affairs for vaccines, Mark Feinberg, said it could be a few years before further investigation and results of other drugmakers' vaccine tests clear up the mystery.
