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VACCINES made from live but weakened HIV could wipe out AIDS in countries
such as Thailand and Zimbabwe within fifty years, computer models suggest. But
there’s a big catch: such vaccines will inevitably kill some of the people
they’re meant to protect.

Live “attenuated” vaccines made from weakened forms of viruses work much
better than vaccines containing inactivated viruses. But they are inherently
risky—sometimes the weakened virus itself causes disease, or reverts to
the wild-type form.

Tests in monkeys suggest live HIV vaccines could work in humans, but their
use is controversial. “HIV strikes at the heart of the immune system. I don’t
think a live attenuated HIV vaccine is analogous to any other live attenuated
vaccine in terms of the risks,” says Marnie Elizaga of the University of
Washington’s Vaccine Trials Unit in Seattle.

So to assess the risks, Sally Blower of the University of California in Los
Angeles and her colleagues modelled the effects such vaccines would have in
Zimbabwe and Thailand. They assumed that live attenuated vaccines would be
between 50 and 95 per cent effective in blocking the spread of wild-type HIV,
and that they would cause AIDS in between 1 and 10 per cent of vaccinated
people.

Blower’s team found that each of the vaccines would eradicate wild-type HIV
in both countries within 50 years and the vaccine strains would simply die out
when vaccination stopped. However, while the model predicted declining death
rates for Zimbabwe, in Thailand most vaccines would initially cause a rise in
deaths from AIDS. That’s because Thailand has a substantially lower HIV
transmission rate, Blower says.

“The important point is that the same vaccine would have different effects in
different countries,” she says. “In developing countries where the transmission
rates are very high, you would have a beneficial effect. But you would be
killing off a certain fraction of the population. If you had these vaccines and
nothing else, yes, you should use them. But if you had something better, then go
with something better,” says Blower.

Not everyone agrees. “I think there is no justification whatsoever under any
circumstance,” says Max Essex, chairman of the Harvard AIDS Institute. He says
that it’s virtually impossible to work out if such vaccines are safe, because
they may not cause AIDS for almost 20 years, and safety trials usually only last
six to eight months. “The most horrible, scary scenario with a live attenuated
HIV vaccine is one where it does not induce disease or death in anyone within 15
or 20 years, but after that induces death in everyone.”