Why is it so hard to develop a vaccine for AIDS? Vaccine development depends on finding a protein on the outer layer of the virus that will serve as a suitable target (antigen) for antibodies produced by the body. In order for the vaccine to work in a predictable way, that antigen should come from a fairly stable gene and not one that is highly susceptible to the many causes of gene mutations.
The human immunodeficiency virus (HIV), the virus that causes AIDS, is an example of a retrovirus. These viruses replicate by reverse transcription of an RNA genome. The problem is that the reverse transcriptase enzyme in HIV is much less (about 10-fold) accurate than other known reverse transcriptases. As a consequence, there is a very high rate of mutation in the HIV genome every time the virus replicates. This makes finding a vaccine a difficult task, which is why last weeks news from the International AIDS Vaccine Initiative (IAVI) was so significant. The drug AZT, that has been in use since 1985, works by competitively inhibiting reverse transcriptase in HIV, and by causing the formation of truncated viral RNA chromosomes.