Three U.N. agencies are launching an international research effort to develop a single-dose "super vaccine" that children could receive soon after birth to protect them against all of the major childhood infections.

The program, sponsored jointly by the World Health Organization (WHO), the United Nations Development Program (UNDP) and the United Nations Children's Fund (UNICEF), is expected to cost at least $150 million over the next decade. It is being announced today in New York at an international meeting on vaccine development.

Enormous gains have been made in the last two decades in immunizing the world's children against potentially fatal infections. About 70 percent of children in developing countries now receive six standard vaccines, preventing an estimated 2.5 million deaths each year, said Ralph H. Henderson, assistant director-general of WHO. The vaccines protect against measles, tetanus, diphtheria, whooping cough, polio and tuberculosis.

But an equal number of deaths are thought to occur each year among the 30 percent of children in developing countries who are not immunized. Henderson said that the key to reaching those children -- the world's poorest and most vulnerable -- is to combine vaccines and to make them more effective, so that a child could be immunized during a single visit to a clinic instead of making four visits during the first year of life.

It may take two or three decades to realize the goal of a single-dose super vaccine, said Barry Bloom, a professor of microbiology at Albert Einstein College of Medicine and a WHO consultant. But he added that the technology is already available to combine vaccines and to make them in a timed-release form. He predicted that by the end of the decade, all of the standard immunizations could be given at a single visit.

"Vaccines are the most cost-effective medical intervention against disease in the world," he said. He added that the new program, which is being announced in anticipation of the U.N.'s World Summit for Children later this month, will "bring the highest power of biotechnology to what many people see as the most vulnerable part of society."

Many of the vaccines currently available require several doses to provide maximum protection. Most are less effective in newborn infants than in older children because an infant's immune system responds less vigorously to foreign organisms and proteins. American children typically visit a pediatrician six times for immunizations by the age of 5.

In addition, three vaccines -- for measles, polio and tuberculosis -- contain live viruses or bacteria and must be refrigerated, which complicates their use in developing countries. And all of the commonly used vaccines except the oral polio vaccine must be given by injection, which requires clean needles and trained health-care workers.

Researchers now have ways to attack all of those problems. Two promising developments make the idea of a super vaccine realistic. First, researchers can "piggyback" components from several different vaccines onto one carrier. Second, vaccine components can be packaged in microcapsules that dissolve slowly after injection, continuously stimulating the immune system for weeks or months. Such timed-release vaccines could eliminate the need for multiple doses.

Bloom said this approach may soon be used to create a single-dose tetanus vaccine that could be given to pregnant women. Antibodies against tetanus made by a woman's immune system would pass through the circulation to her infant, protecting the baby from tetanus after birth. Henderson said tetanus kills an estimated 750,000 newborn infants a year in developing countries.

He said efforts also are underway to develop new vaccines for major childhood killers for which no protection is currently available. These include respiratory syncytial virus, which causes an estimated 3 million deaths a year; rotavirus, a major cause of diarrhea that kills 800,000 children a year; meningococcal meningitis, which kills about 200,000 children a year; and malaria and other parasitic infections.