In a field that has seen more failure than success, experts received the news of an effective new AIDS vaccine with a fair share of skepticism. In September, a $105 million trial of a novel combination of two older vaccines was the first to show protection against HIV infection. The results of the trial, which involved more than 16,000 volunteers, suggested that the vaccine was 31% effective at preventing infection among those who were inoculated. It was a modest outcome, given that behavior-based prevention methods, like condom use, can be equally if not more effective. The volunteers were also largely heterosexual and monogamous, putting them at low-to-moderate risk for HIV infection — rather than high-risk, like intravenous drug users — and prompting questions about how impressive the results of the study really were. But given that no other inoculation has shown any effect against the AIDS virus, it was reason to celebrate — cautiously.