Every two seconds, somewhere in the world, a child dies before reaching its fifth birthday. Two probably died while you were reading that sentence.

An estimated 15 million children under 5 die each year, but world health authorities are zeroing in on another number, smaller but still huge. For them, the number 3.5 million is a symbol of both horror and hope.

That is how many children under age 5 die every year of diseases that are preventable by vaccines. Another 4 million to 5 million children die of fluid loss from diarrhea.

"The numbers are staggering," said Dr. Ralph H. Henderson, director of the World Health Organization's program on immunization. "That's a child's death every time you take a breath."

Henderson and several dozen other doctors and health ministers from around the world spoke last week at the International Vaccine Symposium, a two-day conference of world health leaders at the Pan American Health Organization in Washington.

The World Health Organization's current effort to immunize the world's children, begun a decade ago, focuses on six dread diseases: Measles, which kills about 2 million children a year. An unvaccinated child is almost sure to get measles. Pertussis, or "whooping cough," which infects millions and kills about 600,000 children a year. Neonatal tetanus, which is contracted through contamination of the umbilical cord at birth. It kills nearly 1 million children a year. Polio, which is the leading cause of lameness in children in developing countries. It kills between 30,000 and 50,000 children a year. Tuberculosis, which attacks about 10 million people a year and kills about 50,000. The most severe strains primarily attack children. Diphtheria, which is limited primarily to young children and has a mortality rate of about 10 percent.

All of these diseases share two important characteristics: They kill young children, and they can be prevented by an existing, safe, effective vaccine.

Immunization of the world's children, in a sense, is a solution looking for its problem.

Despite awesome logistical barriers -- ranging from funding and inaccessibility to illiteracy and climate (unrefrigerated vaccines lose potency) -- Henderson said "substantial progress" has been made since WHO stepped up its immunization efforts in 1974. This year, he said, WHO's expanded immunization program will save the lives of about 800,000 children who otherwise would die of measles, neonatal tetanus or pertussis.

"I've spent most of my career in the World Health Organization trying to warn people about how dangerous the situation is," Henderson said. "But now, everybody knows about immunization. We are now as a world committed to immunization in a way the world was not before."

Instead of specific percentage targets for immunization against each disease, WHO has set the general -- and admittedly unrealistic -- goal of universal immunization by 1990.

"We're trying not to get too hung up on a specific numerical goal," Henderson said. "But we're going to fight like hell to get every last child in that system by 1990, knowing that we're not going to make it."

But the challenge is enormous. Only about 40 percent of children in developing nations have been immunized against diphtheria, pertussis and tetanus (with the DPT vaccine) and against polio, according to WHO. Between 20 and 30 percent are protected against measles.

More than 30 years after discovery of the polio vaccine, 6,000 children a week are stricken with polio, said Dr. William H. Foege, special assistant for policy development at the Centers for Disease Control in Atlanta. More than 20 years after a measles vaccine became available, 40,000 children a week die of measles.

"If those of us in this room were forced every Friday afternoon to confront the parents these children and try to explain why," Foege told conferees at last week's vaccine symposium, "we would very quickly run out of excuses." Foege also debunked a persistent myth about childhood immunization: that health officials, by wiping out killer diseases in children, would merely worsen the population problems of already overpopulated Third World nations. Studies by the World Bank and UNICEF show just the opposite, he said. The lower a country's infant mortality rate, the lower its net population increase, because parents become confident of their offspring's survival and tend to have fewer children.

Nor should Americans assume that immunization is a problem only for the rest of the world, Foege said. A decade ago, despite the ready availability of a safe vaccine against polio, only 70 percent of American children were immunized against polio.

Just as elimination of measles was the target that symbolized and mobilized the United States' immunization effort, Foege said, the global effort should adopt as its goal the eradication of polio by the end of this century.

Immunization experts are the first to ac- knowledge that vaccines are not a panacea for world health problems. Not only are vaccines unavailable for many serious diseases, but immunization is only one weapon in the war against malnutrition, unsanitary conditions and illness around the globe.

"Don't get mesmerized by universal vac- cine coverage," Foege urged the conferees. "Get mesmerized by disease control."

"Immunization is primary health care, but it is not everything," agreed Dr. Pablo Martinez, coordinator of the Ecuadorian Ministry of Health's Child Survival Project. "There is a cultural component in all this, "Now, everybody knows about immunization. We are now as a world committed to immunization in a way the world was not before." -- Dr. Ralph H. Henderson, director immunization program World Health Organization and we need to involve other ministries besides health."

The key to achieving universal immunization, Foege said, is the involvement not just of doctors but of school nurses, volunteer groups and parents.

"Let's get used to thinking of this as a social movement, not just a health movement," he said. Until the past few years, noted Dr. June Osborn, dean of the University of Michigan School of Public Health, immunization levels for American school children lagged below 75 percent for diseases such as polio and measles -- even when a safe vaccine was readily available. It was only passage of "no shots-no school" laws requiring immunization for school registration that brought immunization levels near 100 percent for such diseases.

For most people, she said, the idea of a vaccine against a dread disease is almost magical, a marvel of human creativity. But the public's fascination with vaccines often turns to boredom and indifference when it comes to the logistical challenge of delivering vaccines to the world's population.

"It's the big mystery of public health -- why prevention is so much harder than thinking about cure," Osborn said.

The paradox is dramatized by the public reaction to the epidemic of AIDS, or acquired immune deficiency syndrome, she said. The public clamors for discovery of a vaccine as the ultimate solution to AIDS, she said, "at the very time when continued vaccine availability is genuinely threatened by issues of liability, misperception of cost-benefit risk analyses, and individual outrage at putative vaccine injury."

Immunization is one of the cost-benefit bargains of public health, Foege noted. Vaccinating all the children in the world would cost about $10 per child, or about $1 billion a year. That is less than U.S. companies spend to advertise tobacco products, he said, and equivalent to what the governments of the world spend on military arms every 10 hours.

"Our greatest challenge may be fatalism, in ourselves and others," Foege said. "If we would do good in the world, it will have to be done in minute particulars. "It will be a day in, day out, continuing, consuming effort, but it can be done."