Today the world’s Aids vaccine researchers are celebrating the first partial success of a clinical trial, after more than 20 years of frustration and failure. But we must not get carried away with the results of the RV144 trial in Thailand.
The positive results were fairly modest. Although the $105m trial was ambitious in scale, with 16,400 healthy volunteers taking part, the number infected with HIV over the three year period was relatively small: 51 among those who received active vaccine and 74 among those receiving placebo shots.
That amounts to a 31 per cent reduction in infection – statistically significant, in the sense that it is unlikely to have occurred by chance, but not impressive compared to the 70-80 per cent protection provided by most licensed vaccines for other diseases.
The immediate priority is to understand how the two components of the Thai vaccine are working together. Each had failed in smaller clinical trials on its own.
The first component, called Alvac HIV, is a canarypox virus genetically engineered to include three HIV genes; it is designed to stimulate human immune cells to fight HIV. The second, called AidsVax, contains a protein from the surface of HIV; its purpose is to stimulate antibody production.
A big puzzle in the Thai trial results, which the researchers will need to solve, is why the combined vaccine had no effect on the amount of HIV in the blood of volunteers who did become infected. Normally an effective vaccine reduces this “viral load”.
The partial success in Thailand does not mean that other approaches to an HIV vaccine should be abandoned. Even after more than 20 years of failure, there are several novel HIV vaccines under development in the world’s laboratories – based for example on studying those rare individuals who do not develop Aids symptoms despite long-term HIV infection – which will be ready soon for clinical trials. They deserve a chance.