(FORTUNE Magazine) – DEC. 1 WAS WORLD AIDS DAY, AND most of the news was sobering--an estimated three million deaths and 4.8 million new infections last year. But amid the gloom, Gordon Brown, Britain's Chancellor of the Exchequer, made a dramatic announcement that could shift the trajectory of the epidemic. He committed his government, in cooperation with other donors, to purchasing an AIDS vaccine if and when it is developed.

That announcement, along with a similar one by Britain a week earlier regarding malaria, signals a revolution in medical research: Using market incentives to harness the private sector to solve problems for the world's most disadvantaged people in a way that both preserves intellectual- property rights and promotes access to life-saving medicines for the poor.

Why are these commitments revolutionary? Vaccines arguably offer the best hope for conquering those diseases. Unlike anti-retroviral medicines (ARVs), vaccines don't require complex diagnostics and are cheap and easy to administer. Chronically weak health infrastructures combined with tiny health budgets result in only a minute fraction of those with AIDS having access to ARVs. The World Health Organization is struggling to reach a goal of treating three million people with ARVs in the next five years. Compare that with the more than 100 million infants who annually receive basic vaccinations, even in the most remote corners of the world and even amid civil wars.

R&D on vaccines for diseases primarily affecting poor countries is tiny. Of the 1,233 drugs licensed worldwide between 1975 and 1997, only 13 were for tropical diseases, and only four were developed by commercial drugmakers specifically for human tropical diseases. The challenges of developing vaccines for diseases like AIDS and malaria are formidable, but biotech and pharma companies often take on difficult challenges. The problem has been that industry is unlikely to invest in such projects if it has little reason to think it will recoup its costs. That is a real concern: Governments often hold down vaccine prices, and most vaccines are sold to developing countries at pennies per dose.

Market mechanisms are key to overcoming those problems. In high-income countries, new drugs are spurred by a potent combination of economic pushes and pulls. Funding from institutions such as the National Institutes of Health pays for most basic research, and the lure of profits encourages firms to transform basic science into products. For drugs needed in poor countries, push programs such as the International AIDS Vaccine Initiative are now in place. While more push funding would help, a major stumbling block remains the lack of a market incentive to turn basic research into products.

Purchase commitments like Britain's provide the missing pull. They reward success without micromanaging the research process and may mark the start of a trend that could save tens of millions of lives--particularly if private donors take the cue. Picture what might happen if sponsors committed $3 billion to buy a malaria vaccine at $15 a person for the first 200 million people immunized. The commitments can be structured like binding contracts. A pot that size would be comparable with commercial drug sales, and it could save more lives than virtually any other imaginable health expenditure.

Raising purchase commitments on such a scale is not impossible. The initiative is gathering momentum, and the Gates Foundation is backing an analysis of the practical details of putting advance markets in place. We urge the U.S. to join these commitments and to use its influence with the World Bank to encourage it to do so too. If purchase commitments fail to induce the development of new vaccines, no money will be spent. But if they succeed, millions of lives will be saved each year.