"It's in the youngest age groups that you can see things happening soonest," Gregson said.
Declining AIDS rates in Zimbabwe could herald similar changes in other parts of Africa.
Zimbabwe may be a bellwether, Gregson says, because it has a high literacy rate, particularly among young people. That may have given its citizens a head start on AIDS awareness compared to neighboring countries.
"People shouldn't give up hope in other countries," he said. "Hopefully, in three or four years we'll start to see similar declines in other places."
Richard Hayes, of the London School of Hygiene and Tropical Medicine at the University of London, agrees. The Zimbabwe findings, he said, are "very significant."
Hayes is co-author of a commentary that accompanies the Science study.
"It is the first really clear evidence that we are beginning to see some [HIV] reductions in southern Africa," he said. "And [Gregson] has strong evidence not only of a reduction, but also of behavior change. That's really good news."
Hayes cautions, however, against applying the results too broadly.
"We tend to think of Africa as one homogeneous region and the epidemic as the same everywhere," he said. "That is very much not the case."
A few years ago, Hayes was part of a team that carried out its own study of factors influencing HIV infection rates in four African cities.
The researchers expected that the cities where residents practiced the riskiest sexual behaviors would have the highest infection rates. But that wasn't the case.
"That was quite a surprise," Hayes said.
What the study instead found was that circumcised men appear to be less likely to catch the disease than uncircumcised men. (Read a related news article.)
People with other sexually transmitted diseases, particularly herpes, are also more likely to become infected with HIV.
Currently, Hayes says, three studies are looking more closely at circumcision as an HIV prevention measure. One study in South Africa found a two-thirds reduction in the infection rate of recently circumcised men compared to uncircumcised men.
"These studies emphasize that sexual behavior is only one of the factors driving AIDS epidemics in Africa," Hayes said. "Although [behavior] seems to have played a key role in the HIV decrease in Zimbabwe."
Keeping Our Eyes on the Ball
Hayes warns that there is a risk that hopeful findings can cause people to lapse into complacency.
"The temptation is to say that things are looking up a bit," he said. "That would be a dangerous message."
Complacency can lead to a resurgence of risky behavior, he says. For example, the ready availability of AIDS drugs in the U.S. and Europe may be lulling people into thinking they can take more risk.
Furthermore, as the same drugs become prevalent in Africa and the life spans of infected people increase, there will be an increase in AIDS-related medical costs.
"Unless we can turn off the tap of new infections, it will be increasingly difficult to sustain treatment programs," Hayes cautioned.
John Stover, a policy analyst based in Glastonbury, Connecticut, agrees.
Stover works for the Futures Group, a consulting firm that specializes in creating public health programs for developing nations. In a study published this week in the online journal Science Express, his team compares the cost of preventing HIV infections to that of treating them.
Some prevention programs are easy and inexpensive, he says, such as focused intervention programs in settings with a high occurrence of HIV. But others can be costly.
Nevertheless, Stover's team concluded that a comprehensive package of prevention programseven including the most expensive planswould cost up to U.S. $800 less per case than treatment.
That's an important message, Stover says, because treatment programs tend to hog media attention.
"We want to remind people that prevention is important and affordable as well," he said.
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