Scientists at the Centre for Applied Microbiology Research at Porton Down, Wiltshire, found that an experiment with a prototype vaccine against monkey Aids showed how easily a supposedly safe vaccine can change into a dangerous form. Using a live monkey Aids virus as a vaccine, which had been 'attenuated' or crippled to make it harmless, they found that in one out of eight macaque monkeys in the experiment the virus repaired itself within 17 weeks of being injected into the animal.
The virus went on to make further repairs over the next 28 weeks until it became almost indistinguishable from the virus that causes Aids in monkeys. The scientists found that the virus 'vaccine' had replicated freely within the inoculated animal and had reverted to its dangerous form.
Live attenuated viruses are used as vaccines against infections ranging from measles to polio. Many Aids researchers believe they offer the best chance of a vaccine against Aids. Previous research on the simian immunodeficiency virus, SIV, showed it was possible to use live attenuated forms of the virus as a vaccine. Ronald Desrossier at the New England Regional Primate Research Centre near Boston, Massachusetts, found in December 1992 that attenuated SIV gave monkeys the best long-term protection against infection with the virulent forms of the virus.
After showing that monkeys inoculated with the vaccine could survive repeated injections of dangerous strains of SIV, Professor Desrossier proposed that human trials with an attenuated HIV vaccine should begin as soon as possible.
But Professor Jim Neil, an Aids researcher at Glasgow University, said the British research demonstrated the potential dangers of using live attenuated viruses as vaccines. A potential vaccine that proves to be safe under laboratory conditions may evolve into dangerous forms when used in the outside world, he added.