Measles is virtually nonexistent. Polio is all but wiped out. A number of childhood diseases, some with potentially crippling effects, no longer are serious health problems.

That is the near-miraculous good news of a campaign, led and financed by the federal government, to get America's children immunized against these once-prevalent diseases.

The bad news is that the Reagan administration now proposes to cut back on the immunization program. It won't appear on the books as a cut: in these inflationary times, few things do. In fact, the president's fiscal 1983 budget calls for a modest increase in immunization funds over last year--$29 million as opposed to $28.3 million. But health officials say the $29 million would leave them about $5 million short of what is needed to immunize the same number of children as in fiscal 1981, when the immunization budget was $30.9 million.

In any case, we're talking peanuts: molehills of expenditures for mountains of benefits, including money benefits. For not only do the immunizations prevent the pain and suffering of the childhood diseases; they are also far, far cheaper than the cost of treating the diseases.

Administration officials insist that it isn't just false economy that has produced the proposed cutback in immunizations. Precisely because the program has been so successful, they say, nearly all America's children are immunized already, and the additional money simply isn't needed.

There's no questioning the success of the program. There were 21,000 cases of polio in 1952. Last year there were six.

But as Jane Stein of the National Journal notes, "The heart of the cost issue is the problem of the hard core of low-income children who don't receive immunizations. . . . These children are difficult and expensive to reach because they require tracking down by school nurses and public health authorities."

As for the notion that the program has been so successful that only a modest effort is needed to sustain it now, Stein makes this point:

Between 1966 and 1968, the government spent $21 million on measles vaccinations, and the number of reported cases of measles dropped from 160,000 a year to only 22,000. Then in 1969, when a specific vaccine for rubella (German measles) was licensed, the government decided to put all its money into the rubella vaccinations. "With the public coffers bare," says Stein, "immunity against (other forms of) measles fell, and the number of cases rose to 70,000 by 1971."

Measles incidence fell back to 30,000 in 1971 after the government resumed its immunization program.

The National Journal reported a similar trend in the case of polio vaccinations. At one point, as much as 90 percent of the population had received the vaccine. Then government outlays for the polio immunizations fell to some $6 million a year, and the percentage of Americans who were protected from the disease fell to between 65 and 75 percent. Alarmed federal officials increased their outlays to $17 million, and immunization levels soared.

The whole sequence is entirely predictable, according to H. David Banta, a health official with the congressional Office of Technology Assessment. "When federal investment in vaccines falls, the doses administered in this country fall." And when the dosages decrease, the incidence of the diseases increases.

If Reagan's budget people don't understand this, they ought to talk to the people at the Center for Disease Control in Atlanta, where they have been pleading for an extra $5 million or so: a few million dollars to prevent diseases that might require a few billions in medical care.

As the oil-filter man puts it, we can pay now or we can pay later.