What is HIV?
For an explanation of terms used, please reference our Glossary of Terms.HIV (human immunodeficiency virus) is a retrovirus that carries its genetic code in the form of RNA (ribonucleic acid). Retroviruses use RNA and the reverse transcriptase enzyme to create DNA (deoxyribonucleic acid) from the RNA template. The HIV virus invades a human cell and produces its viral DNA which is subsequently inserted into the genetic material (chromosomes) of the cell. This infection converts helper T-cells (a type of white blood cell) from immunity producing cells into cells that produce and release HIV virus particles into the blood stream destroying the immune defense system of the individual.
Several AIDS-causing HIV-1 virus subtypes, or "clades", exist in different regions throughout the world. These subtypes are identified as subtype A, subtype B on through C, D, E, F, etc. The predominant subtype found in the European Union, North America, South America, Japan and Australia is B; whereas the predominant subtypes in Africa are A and C. In India, the subtype is C. Each subtype is at least 20% different in its genetic sequence from other subtypes. This difference may cause a vaccine, known to be effective against one subtype, unsuccessful against other subtypes.
HIV-1, even within subtypes, has a high rate of variation or mutation. In drug treatment programs, virus mutation can result in virus escape rendering drug therapy ineffective. Hence, multi-drug therapy is very important. If several drugs are active against virus replication, the virus must undergo multiple simultaneous mutations to escape. An action which is very unlikely. The same is true for immune responses. HIV-1 can escape simple immune responses. However, if an immune response is directed against multiple targets (epitopes), virus escape is much less frequent. Vaccination against more that one of the proteins found in HIV-1 virus maximizes the number of targets for the immune response and increases the chance HIV will not escape the vaccine-stimulated immune response, thus resulting in protection against clinical AIDS.