As the world struggles to fend off mysterious and complex plagues like AIDS and malaria, it must still find a sustainable way to ensure that children do not die from diseases that vaccines can prevent. Unhappily, this year the supply of vaccines against diphtheria, tetanus and pertussis will fall short of demand in many nations. Yellow fever and oral polio vaccines suffered recent scarcities.

In general, the market has tightened for most vaccines used in developing countries. Universal childhood immunization, a campaign begun in 1985 by Unicef and the World Health Organization, has been one of the world's most significant health achievements.

By the early 1990's, more than 70 percent of children in developing countries got basic vaccines, up from zero in some parts of Africa. But now progress has stalled, and Africa is slipping backward.

One-quarter of the world's children get no protection from the diseases that basic vaccines can ward off, and two million children will die of these diseases each year. One of the reasons is AIDS, which is killing off the health workers who administer vaccines and is hogging the health care dollars of African nations.

The debt crisis is also reducing African health care budgets. A particularly vexing reason for vaccines failing children in poor countries is that they are protecting children in rich ones increasingly well. New vaccines and combinations used in the developed world are too expensive for poor countries, but since the old versions are used mainly in nations that cannot pay, manufacturers have little incentive to manufacture them.

The Global Alliance for Vaccines and Immunizations, created with an initial grant from the Bill and Melinda Gates Foundation in 1999 as a partnership of international organizations, governments, philanthropies and vaccine manufacturers, has helped to mobilize global support for immunization. It also helps the poorest nations extend their vaccine coverage, and through Unicef buys them new vaccines. The alliance is hoping to provide manufacturers with a steady, predictable demand for vaccines for poor nations.

Developing nations can do a better job of forecasting their needs. Wealthy nations that underwrite immunization programs need to think in the long term, pledging their donations five years in advance. The business of vaccines is changing. Predictability is now the key to insuring that the gap between rich children and poor does not continue to widen.