THE RESULTS of the Thai Aids vaccine trial, while encouraging, must be treated with caution. They follow over 20 years of failed scientific trials seeking to find a vaccine against HIV, the virus that causes Aids.
With just over 16,000 subjects – one half of whom were given the experimental vaccine, and the other half a placebo – the trial is of moderate size. Participants were followed up over a three-year period and were tested for the HIV virus every six months. At the trial’s conclusion, some 51 of the 8,197 people given the vaccine developed HIV, while 74 of the 8,198 who received placebo shots contracted the infection. Although statistically significant, this 30 per cent lower risk of infection is insufficient to warrant the vaccine’s introduction to everyday practice.
A number of other caveats have also emerged. Because there are different sub-strains of the HIV virus active in different regions, all that can be said of this trial is that the combined vaccine is partially effective against the HIV strain prevalent in southeast Asia. In particular, the many people at risk of Aids in Africa would be unlikely to benefit. Another unusual feature is this vaccine’s failure to reduce the amount of virus in the victim’s blood. Usually vaccines, even when only partially effective, will cause the amount of circulating virus to drop, thus rendering the person less infectious to others. This finding raises questions about how the new vaccine stimulates the immune system – a key aspect of how immunisation works.
It is also significant that previous trials of the individual component vaccine, ALVAC and AIDSVAX, showed no benefit.
Surprisingly, too, the results have been released in a rather ad hoc fashion. Major scientific trials are usually published in reputable medical journals, often at the same time they are released at a large medical conference. Apparently, more detailed results are due to emerge at a Paris vaccines conference next month. However, yesterday’s premature release will cause some eyebrows to be raised in the usually staid scientific and medical communities.
Perhaps the best that can be said about these trial results is that they will reinvigorate the research community in the belief that an Aids vaccine is once again a real possibility.