The principle behind vaccination is remarkably simple. Present a harmless version of the microorganism to the immune system and it will be duped into a state of readiness. Should the genuine article come along, the foreign invader will be dealt with swiftly and effectively. Such approaches have worked well with other viral diseases such as smallpox, polio, and measles. Either attenuated or killed viruses may be used. Attenuated virus seem to elicit better immune responses and is the vaccine of choice when possible.
Concerning HIV, however, scientists are very reluctant to inject healthy people with weakened yet alive virus. There is too great a danger that HIV will mutate to a more lethal or virulent form. Researchers dare not use even a killed organism for fear that some viral particles may have survived the killing process. Even a single surviving fragment can invade a cell and initiate a cycle of infection.
Even if an attenuated HIV does not revert to virulence, there is always the possibility that it can reacquire the ability to integrate into the host genome. No one knows what the long term effect of integrated HIV might have on a cell. With other human retroviruses, it often leads to cancer. Researchers do not want to produce a vaccine that causes malignancy ten or fifteen years down the road.
To appreciate the difficulty in producing an HIV vaccine, consider the influenza virus, which has a mutation rate just slightly less than that of HIV. Each new flu season holds the often fulfilled promise of epidemic influenza against which the previous year's vaccines are ineffective. Unlike HIV, the dangers of killed influenza vaccines are relatively low.
Vaccines for influenza are quite straightforward: Identify the epidemic type, kill it, and circulate it as a vaccine to combat the epidemic. (My note: the changing influenza virus is evolution in action at a rapid pace. Keep that in mind, for it illustrates how diseases change over time. They respond to environmental conditions--their ecological setting.)
HIV is a retrovirus, a type of infectious agent that have the alarming ability to integrate their own genes into the DNA of the cells they infect. In other words, once a retrovirus infects a cell, it becomes part of the nuclear DNA and can be passed onto succeeding generations. This in itself makes vaccine development extremely risky. .....
A weakened strain of SIV (simian) that protected adult monkeys, for example, looked safe until researchers at the Dana Farber Cancer Institute in Boston showed that newborn monkeys with immature immune systems did not respond as healthy adults do. All the young primates, in fact, developed the very disease the weakened virus was supposed to prevent.
The rapid mutation rate happens because the reverse transcriptionase enzyme that triggers the reproduction of the virus's RNA is not very efficient. It makes many 'mistakes,' thus making virus populations with a huge variety of genetic traits.
This burst of activity seems to occur to ensure that there are enough viruses capable of surviving and reproducing within the host's body. Every virus that breaks out of one cell and goes in search of another to colonize will mutate just as much once it has wormed its way in and begins to reproduce.
The variety of mutants produced in each generation means that a vaccine tailored to deal with one person's HIV is unlikely to create antibodies that work on another's virus population.
..... CJ '99
Resources
Brooksmith, P. Biohazard The Hot Zone and Beyond New York: Barnes and Noble Books, 1997.
Nash, J. 'An AIDS Mystery Solved' Time November 20th, 1995.
Zimmerman and Zimmerman Killer Germs. Chicago: Contemporary Books, 1996.