Figures from the Joint United Nations Programme on human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS) suggest that 40 million people are infected with HIV worldwide. Thus the need for an effective HIV vaccine is more pressing than ever. For individuals already infected, potent drugs are available to control the virus. However, these drugs often have significant toxicities and are difficult to afford in many parts of the world, particularly where they are most urgently needed. In addition, noncompliance with drug regimens is believed to contribute to the development of drug-resistant strains of HIV. Hence a vaccine remains the best hope of controlling the HIV pandemic.