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The life of prostitutes in Pumwani is hard. When they contract HIV, they progress to full-blown AIDS in an average of just four yearsfar less time than Kenyan women who aren't sex workers, not to mention First World women. The prostitutes, says Plummer, live "incredibly violent" lives. In 1996, Selina was raped repeatedly. At that point she had tested HIV-negative for 11 years. But shortly after the rape, she tested positive. She has already suffered a variety of AIDS-related illnesses and lost more than 10 percent of her body weight. The veins on her forearms are prominent, running like long ridges down the smooth landscape of her skin. Yet Selina has staunchly refused to be told the results of her test. Kimani explains, "She can't come to terms with the fact that she thought she couldn't get HIV and now she has."
There are a few others like Selinamultiply exposed, persistently negative women who suddenly have become positive. Plummer and Kimani think that in Selina's case, the stress of the rape might have weakened her immune system. But the other women who have belatedly turned positive seem to have another risk factor: stopping prostitution.
High levels of killer T-cells are not normally maintained for long periods. They arise in response to an invading microbe, then subside. So perhaps what's keeping the women immune is constant, low-level exposure to HIV from their johns. When the women take a vacation, their killer T-cells wane, leaving them vulnerable to the virus.
What does this mean for a vaccine? "It's not good news," says Plummer. After all, one would hope that immunity lasted forever, or at least for many years. If frequent booster shots are required, a vaccine would be unaffordable where it's needed most: in the developing world.
But McMichael has a different take on the newly infected women. Kimani found that resistance runs in families, suggesting a genetic trait. Such traits have been found in other people; some Caucasians, for example, have a mutation which makes their cells impregnable to the most common strains of HIV. But the fact that the Pumwani women can get infected is actually "good news," says McMichael. "Not for those women, of course, but it is good news for a vaccine because it means they don't have some special, undetected genetic immunity that a vaccine couldn't induce."
By far the biggest obstacle to an AIDS vaccine is the virus itself. For one thing, HIV has many different strains, called subtypes, and the major one in the United States and Europeand hence the one most used in most candidate vaccinesis different from the dominant subtypes in Africa. No one knows if a vaccine designed against one subtype will protect against infection by a different one.
Then too, people's immune systems are different, composed of various "HLA types." Even if infected with the same strain, people often display different viral epitopes on the surface of their cells. These differences tend to correspond with ethnicity, so a vaccine that protects most whites might not be as effective among Asians or Africans.
McMichael has tackled these problems by making his vaccine out of fragments of the HIV subtype most common in Kenya, subtype A. In addition, he has included enough epitopes to cover probably every HLA type in East Africa. Still, McMichael's vaccine contains only 44 epitopes, plus a whole viral gene. Are these enough? And are killer T-cells truly the key to protection?
Most of the uninfected prostitutes appear to produce a special antibody in the place where HIV first enters them: the mucosal tissue of their vaginas. So, should a vaccine induce site-specific antibodies? There are other experimental vaccines that attempt to do exactly that.
No one knows the answers to these questions, but Africans are not waiting for the West to solve the problem. "I lost my own brother to AIDS two years ago," says Anzala, who has dedicated many years to studying the Pumwani sex workers' immune systems. "That really devastated me, because with all the knowledge I had. . . . " His voice trails off; then he snaps back and adds, "We can't wait for something to come from the U.S. No. We have to participate."
Indeed, African scientists are actively contributing to the research, providing ideas and laboratory research, pushing scientists to put candidate vaccines into trials on their continent, and insisting that the world develop vaccines that are likely to work against their subtypes of HIV. Uganda is already hosting the continent's first human trial of an AIDS vaccine, and Ugandan researchers are conducting the sophisticated laboratory analyses needed to evaluate the trial. South Africawhich accomplished the world's first successful heart transplanthas the most developed biomedical research capacity in Africa. President Thabo Mbeki has declared an AIDS vaccine a top priority, committing government funds to a soup-to-nuts research effort. "We're not just trial sites in Africa," says Quarraisha Abdool-Karim, a veteran South African AIDS researcher. "We have an intellectual contribution to make."
While the Africans are pushing as hard as they can, AIDS scientists around the world are also putting vaccines high on the agenda. The U.S. National Institutes of Health, which dwarfs any other medical research agency in the world, and which spends more than a billion dollars on AIDS research alone, used to give vaccine research less than 10 percent of its AIDS budget, less than any other category of HIV research. But over the last three years it has ratcheted up that percentage, and it has brought in Nobel laureate David Baltimore to lead its effort. Whereas the mood was once pessimistic, most scientists now believe a vaccine is possible.