It's always being said about the United Nations that, however vexing its political forums, the specialized agencies do vital work. It's often true, and some of that work comes into deserved focus Thursday. As part of the world organization's 40th anniversary, the secretary general has arranged to have the General Assembly give a ceremonial boost to the campaign to get every member state to immunize all its children against diphtheria, tuberculosis, poliomyelitis, whooping cough, measles and tetanus by 1990. The six diseases are estimated to take millions of lives a year, and they hover over families in the Third World like a dark cloud.

The startling thing about the immunization campaign, launched in 1974, is how quickly it has become feasible to end a scourge that has marked the whole of man's recorded history. Just in the last few years the requisite medical, informational and organizational resources have become widely enough available to make possible the provision of life-saving services to people and to sectors of society who never previously thought to expect them.

Much needs to be done both to spread an awareness among parents that their children can be saved and to put together the vaccines, trained personnel and the local networks of primary health care. Much is being done, however, by the World Health Organization and the United Nations Children's Fund and such regional bodies as the Pan American Health Organization, by public authorities drawn by the political benefits as well as the social benefits of caring for their people (El Salvador broke off its war for three days earlier this year to let the children be immunized), and by the providers of the funds. Up front is the immunization of children. Behind is the strategy of using that popular cause as the peg on which to organize comprehensive and continuing health care for people who have not had it.

Inevitably, the poorer and more debt-ridden countries face special difficulties in applying their scarce energies and foreign exchange to a program whose principal benefits flow to people with little political power. The International Monetary Fund, the World Bank and other funding sources must do their part to ensure that crucial financial adjustments are not made at the expense of the poor. Horn-tooting sessions like the one planned tomorrow at the U.N. can help toughen up the politicians who are on the front line.