Vaccines usually consist of inactivated viruses that prompt the immune system to remember the invader and launch a strong defense if it later encounters the real thing. However, this approach can be too risky with certain viruses, including HIV. In recent years, many scientists have been exploring DNA as a potential alternative vaccine. About 20 years ago, DNA coding for viral proteins was found to induce strong immune responses in rodents, but so far, tests in humans have failed to duplicate that success.
