Researchers have been working on an HIV vaccine since the 1980s, but progress has been much slower than anticipated.
One reason is the speed at which HIV mutates. Most vaccines against other diseases stimulate the production of antibodies. These antibodies ‘neutralise’ the virus. But in the case of HIV, neutralising antibodies do not clear the infection.
This is because HIV reproduces so fast and mutates so quickly. Antibodies produced against the virus quickly become ineffective against newer viruses. Millions of new viruses are produced each day and each one is slightly different from previous generations of the virus. Antibodies against HIV are only likely to be effective if they can bind to regions of the virus that vary little between viruses.
Another reason is that there are several sub-types of HIV. Each one is concentrated a particular region of the world. A vaccine must be effective against all sub-types, or different vaccines must be developed against various sub-types.