Only a vaccine can end the AIDS epidemic.

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The powerful new AIDS drugs, besides being too expensive for developing countries, do not cure the disease. In America and Europe, drug resistance and severe side effects are undermining the treatment of more and more patients, and the notion that HIV could be purged from the body has been shattered. The virus, which integrates into a patient’s own DNA, appears to persist for life.

In theory, behavioral changes could stop the epidemic, and many Africans look to Uganda for hope. Ugandan president Yoweri Museveni aggressively confronted the epidemic, and infection rates in some urban areas have declined dramatically since the early 1990s; one surveillance site found that the prevalence of the virus has fallen by half. Yet even at that site, more than 13 percent of pregnant women are still infected — a huge pool of HIV-positive people. While education certainly can save millions, the fact is that behavioral change has never managed to halt the epidemic, not even in wealthy countries.

But vaccination has eradicated one disease — smallpox — and is on the verge of eliminating a second — polio. Ugandan researcher Roy Mugerwa, principal investigator of Africa’s first AIDS vaccine trial, says, “We have learned from history that the only way to halt epidemics is with a vaccine.”

Vaccines do not fight off infection; instead, they teach the immune system to recognize and attack the microbe. The world’s first vaccine, for smallpox, was the cowpox virus, which causes only mild symptoms in people but primes the immune system for smallpox. Salk’s polio vaccine was simply a killed polio virus. Technology has advanced, but the principle remains the same as when the ancient Chinese used to blow pulverized smallpox scabs through a bone into people’s noses: Train the immune system with a dummy virus.

But can the body be taught to fight off HIV? There was a time when many scientists came close to despair, and many still harbor doubts. After all, AIDS attacks the immune system itself, and it kills almost everyone it infects. There were always people who recovered from smallpox, and there were many more who never even showed symptoms because they fought off the virus so quickly. But the more scientists learned about the natural history of AIDS, the more it seemed that everyone infected would succumb and that no one could repel the virus.

This is why the Pumwani prostitutes are so important. It’s also why “people didn’t believe us in the beginning,” recalls Omu Anzala, one of the researchers who studied the Pumwani women. Had the sex workers really come in contact with the virus? They certainly didn’t test antibody-positive, the classic trace of an infection. So maybe, despite all their johns, they had never encountered the virus.

But the virus leaves other traces. The immune system has two main arms: antibodies, which attack viruses floating free in the bloodstream, and cytotoxic T-lymphocytes — or killer T-cells — which destroy the body’s own cells that have been infected. Like antibodies, killer T-cells are specific to one microbe, so they, too, are a kind of fingerprint.

What happens is that an infected cell displays on its outer membrane fragments of the virus called epitopes. Killer T-cells that recognize these particular epitopes destroy the infected cell. What’s more, the immune system clones millions of killer T-cells that are specific to those epitopes, in order to wipe out all the cells the virus has infected. So, high numbers of HIV-specific T-cells indicate that the virus was present.

Oxford researcher Andrew McMichael is one of the world’s leading experts on the killer T-cell. McMichael and his colleague Sarah Rowland-Jones had studied multiply exposed yet uninfected sex workers in the West African nation of the Gambia, and many of them had elevated numbers of HIV-specific killer T-cells. But in the world of the multiply exposed, the Gambian sex workers couldn’t hold a candle to the Pumwani prostitutes. They were the acid test.

Plummer’s team had found evidence of HIV-specific killer T-cells, but many scientists weren’t convinced. Collaborating with Plummer, the Oxford researchers verified the findings, banishing virtually all scientific doubt: The women really had been exposed to HIV, and their bodies had mounted a defense with killer T-cells.

Could those cells be the key to protection?

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