After more than two decades of relentless effort and several failures, there is a glimmer of hope, from the results of the Phase III HIV/AIDS vaccine clinical trial conducted in Thailand. The vaccine candidates used in the prime-boost trial that started six years ago and involved more than 16,000 volunteers in the 18-30 age group have been found to be safe and 31 per cent effective in preventing HIV infection. Compared to vaccines against other diseases, the efficacy of this vaccine is just modest. Yet, this is the first time that a vaccine to prevent HIV infection has reached this level of effectiveness. The two vaccine candidates — ALVAC-HIV and AIDSVAX — used in this trial had earlier failed in their objectives when tested individually on humans. That led to the idea of using two different candidates in a prime-boost vaccine strategy in fighting the virus. The Phase I AIDS vaccine trial under way at the Tuberculosis Research Centre, Chennai, and National AIDS Research Institute, Pune, is using such an approach. But unlike in the Thailand trial, the booster used in the trial in the two cities evoked very good immune response during the Phase-I trial last year at TRC.
While showing modest effectiveness in preventing HIV infection, the vaccines tested in Thailand failed in their second objective of reducing the viral load in those who had become infected. The reason for this is not known. There are several other important parameters that are not known as, for instance, the duration of immune response and the mechanisms by which the two vaccines together prevented the virus from infecting the volunteers. While the duration of protection will become known when the full results are presented next month at an Aids conference in Paris, it will take some time to know how exactly the vaccines work to protect against infection. The information will help researchers in designing prime-boost vaccine combinations that will be more effective and also the type of vaccines that will work in Africa, where the strain is different and the need is the greatest. Despite the modest protection offered, the vaccine will go a long way in bringing down the number of people who get infected every year. Since it is only partially effective, and drugs to clear the infection are currently not available, those who have been administered the vaccine should continue to use the time-tested preventive measures to reduce the risk of infection.