A new experimental vaccine has shown promise in protecting monkeys from a virus similar to the AIDS virus, according to two studies reported in the journal Science. About half the monkeys in the study group were shielded from the virus by the two-step vaccine, scientists said. "I think it is quite promising," said Mitchell Warren of the HIV vaccine and treatment advocacy group AVAC. "In AVAC's 20-year history we have seen a bunch of products never get to efficacy trials. Those that have have mostly failed."