A number of HIV attributes and its mode of infection conspire to render creation of an effective vaccine less than straightforward. These factors include:
- HIV displays extensive genetic variation, often even within a single individual. Such genetic variation is particularly prominent in the viral env gene whose product, gp160, is subsequently proteolytically processed, yielding gp120 and gp41.
- HIV infects and destroys T-helper lymphocytes, i.e. it directly attacks an essential component of the immune system itself.
- Although infected individuals display a wide spectrum of antiviral immunoresponses, these ultimately fail to kill the virus. A deeper understanding of what immunity elements are most effective in combating HIV infection is required.
- After initial virulence subsides, large quantity of cells harbour unexpressed proviral DNA.
The immune system has no method of identifying such cells. An effective vaccine must thus induce the immune system to:
(a) bring the viral infection under control before cellular infection occurs; or
(b) destroy cells once they begin to produce viral particles and destroy the viral particles released. - The infection may often be spread, not via transmission of free viral particles, but via direct transmission of infected cells harbouring the proviral DNA.
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