A new model was developed to examine the potential impacts of an AIDS vaccine in developing countries. The findings suggest that even a modestly efficacious first-generation vaccine could have a profound effect on the AIDS pandemic. A vaccine with 50 percent efficacy provided to 30 percent of the population would reduce new annual infections by 34 percent (seventeen million infections avoided) over fifteen years and result in substantial financial savings. A more efficacious vaccine, combined with expanded delivery, would do even more to control the pandemic. It therefore makes sense to continue investing in AIDS vaccine research and development and the eventual manufacture and widespread distribution of a vaccine.