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FEARS that HIV vaccines may encourage infection with the virus have largely been laid to rest. This could revive the search for a vaccine.

In September 2007, pharmaceutical company Merck halted its STEP trial of an HIV vaccine because it appeared to work no better than a placebo. Then it emerged that recipients of the vaccine who had previously caught the cold virus were more likely than those who hadn鈥檛 to be infected with HIV, sparking fears that the vaccine made HIV infection more likely.

The vaccine consisted of a deactivated cold virus loaded with three HIV genes that were supposed to stimulate immunity. The fear was that participants with immune systems primed to fight the cold virus produced a surge of CD4 cells following injection with the vaccine, and that these cells provided ideal targets for HIV if the recipient was subsequently infected by the virus. 鈥淭he vaccine may have added fuel to the fire,鈥� says in Boston.

Now two analyses of saved blood taken from people injected with the vaccine during preliminary safety studies suggest this isn鈥檛 what happened. Barouch鈥檚 team found that CD4 cells were no more abundant in those who already had antibodies to the cold virus than in those who didn鈥檛 (Nature Medicine, ). Another group, headed by of the University of Pennsylvania School of Medicine in Philadelphia, also tested the blood samples and came to the same conclusion (Nature Medicine, ).

Neither research team can explain why people with the cold virus antibodies were more likely to be infected with HIV. But , a virologist at the University of Oxford, thinks this arose by chance, and that the vaccine failed simply because it didn鈥檛 provoke strong enough immunity to HIV. The work should ease fears over other , says Barouch.

鈥淭he vaccine may have failed because it didn鈥檛 provoke strong enough immunity to HIV鈥�