The search for an effective AIDS vaccine began a quarter-century ago and after a series of failures, there finally appears some reason to cheer. The first signs of success are beginning to emerge, ironically, from a trial (RV144) that seemed destined to flop. The two vaccine candidates — ALVAC-HIV and AIDSVAX — used in the prime-boost trial conducted in Thailand from 2003 to 2009 failed in their prime objectives when each vaccine was tested individually. However, the trial, which involved more than 16,000 healthy volunteers, showed a statistically significant protection rate of 31 per cent in one of the three analyses performed. Further analysis of the trial data has revealed something more interesting. During an AIDS Vaccine Conference held recently in Bangkok, scientists reported the discovery of molecular clues that might have played a crucial role in the vaccine’s performance. Forty-one volunteers who received the vaccine and contracted HIV, and 205 others who received the vaccine but did not get infected with the virus, were chosen for some detailed investigations. Much to their surprise, the scientists found distinct antibodies that provided protection against or made the volunteers vulnerable to HIV infection. A Y-shaped immune molecule called an immunoglobulin G (IgG) was found in the blood of those who were not infected. It turned out that this molecule had the ability to recognise a particular portion of HIV’s outer covering and hence was able to confer better protection. On the other hand, another antibody (IgA) that recognised different portions of the HIV’s outer covering made people more susceptible. According to Nature, these people were 54 per cent more likely to become infected than those who did not have this antibody.

This is the first time a clinical trial has provided interesting leads that hold the promise of advancing AIDS vaccine research. Scientists are already planning animal studies to test the effectiveness of these antibodies. Three human trials using the same vaccines are also likely to start on a small scale in Thailand next year. The results from such studies may tell us if the antibodies caused the protection against HIV or if other factors played a role. But one thing is certain — the outer covering that conferred protection will become one of the most studied aspects of the virus. Only further trials in different populations can ascertain if the antibodies will be able to confer the same level of protection when used in other HIV vaccines and against other HIV strains. There is a ray of hope, but the need to temper optimism with caution stands out in a field littered with failures and disappointments.

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