Although advances have been made in both prevention and treatment, the best hope for prevention and control of HIV/AIDS lies in the development, licensing and delivery of a safe and effective preventive vaccine.
The existence of globally diverse strains of HIV remains one of the greatest obstacles to HIV vaccine development. Attempts to design immunogens capable of eliciting effective neutralizing antibodies against those strains have been unsuccessful. In the absence of such immunogens, the main focus has been on developing vaccines to elicit cell-mediated immunity against HIV. This type of vaccine could suppress viral load, slow the progression of disease, and potentially blunt transmission (15).
The design of improved next-generation candidate HIV vaccines faces many scientific challenges. The mechanisms for protective immunity are unknown, as are the necessary antigens. Despite the failure of one of the most hopeful candidates in recent efficacy trials, however, it is now clear that circulating HIV strains can in fact be neutralized. Another challenge is the extensive genetic diversity of HIV, suggesting that successful vaccines may need to contain cocktails of antigens from across different clades of the virus. Recent clinical data on some vaccines currently in clinical trials have been encouraging, but whether the responses elicited by the vaccines correlate with clinical protection awaits human efficacy trials.
The development of an HIV vaccine faces hurdles of manufacturing, clinical trials, regulation and delivery. These aspects will need to be tackled to ensure that safe and effective HIV vaccines are licensed and delivered as quickly as possible. The development of vaccines is hampered by the limited capacity for efficacy trials, particularly in the developing world. There is also limited regulatory capacity to facilitate the testing and eventual licensure of successfully developed HIV vaccines.
The next five years will probably see the first results from efficacy trials testing the concept of vaccines that aim to suppress viral load, slow disease and potentially blunt transmission of HIV. Several new candidates are expected to be evaluated in clinical trials for safety and immunogenicity. It is likely, however, that to achieve significant improvements in HIV vaccine design and improve the prospects for success, solutions to the major scientific questions will need to be found. As a result, the major stakeholders in HIV vaccine development have recently come together to propose a "global enterprise" to accelerate HIV vaccine development (16). Achievement of this vision will probably require significantly greater resources. The International AIDS Vaccine Initiative recently found that worldwide expenditure for research on HIV vaccines was, in 2001, between US$ 500 million and US$ 600 million, which represented only 10% of the expenditure for research in other areas of HIV/AIDS. Creative strategies to enhance coordination and collaboration among the many stakeholders are also required.