In a best-case scenario, the drug could prevent three million new HIV cases in this part of Africa over a 10-year span, even if it was only made available to the most sexually active individuals, the investigators said.
"This could represent another tool in our arsenal against HIV infection," said Ume Abbas, an assistant professor of medicine at the University of Pittsburgh School of Medicine and lead author of the paper.
The drug in question is tenofovir, one of the cocktail of antiretroviral medications given to HIV patients.
In studies on monkeys, the drug has been shown to be very effective in protecting them against the simian version of the human immunodeficiency virus that can lead to AIDS, and it is now being tested in humans.
US health authorities are funding five separate trials involving high risk groups such as gay and bisexual men, sex workers and intravenous drug users, on four continents.
The earliest results of those trials will not be available until early 2008, but US and British researchers decided to use computer modeling to project ahead to evaluate just how useful the drug treatment might be in reducing HIV transmission rates in the context of the HIV/AIDS epidemic of southern sub-Saharan Africa if and when it is approved for that purpose.
The researchers looked at three different scenarios. In the first they assumed that the drug was effective 90 percent of the time, and that 75 percent of the sexually active population (15-49 year-olds) could be persuaded to pop a daily pill to protect themselves from HIV.
If that rosy scenario panned out, the strategy could potentially cut new HIV infections by a whopping 74 percent over a decade, according to the computer projections.
If the drug was only effective 60 percent of the time and used by just 50 percent of the sexually active population, the reduction fell to about 25 percent over the same time period.
Finally, the researchers modeled a scenario where the drug was effective 30 percent of the time and only a quarter of the target population used it, yielding a reduction in new cases of a mere 3.3 percent.
Even assuming that the drug does prove as effective in humans as it was in monkeys in protecting healthy individuals from infection, it is "never going to be feasible to treat the entire population," Abbas noted.
But even if governments or aid agencies were able to find the funds to supply the drug to the most sexually active individuals -- an estimated 18 percent of the population -- it could still make a big dent in the problem, slashing the infection rate by almost 30 percent over a decade.
That translates to 3.2 million cases.
"Our data highlights the enormous potential public health benefit of pre-exposure chemoprophylaxis against HIV, provided the regimen is efficacious and used consistently daily for a number of years," said John Mellors, a professor at the University of Pittsburgh.
Sub-Saharan Africa is the epicenter of the global HIV/AIDS epidemic, with more than 22 million adults infected with the virus.
The study appears in PLoS One, a journal of the Public Library of Science.