WHILE THE COUNTRY keeps spending more on medical insurance, both public and private, the number of people -- especially children -- with no coverage at all is rising. That unhappy paradox is pointed out by Paul G. Rogers, the former congressman, who is now helping to run the National Leadership Commission on Health Care.

The gradual movement toward universal health coverage was stopped in its tracks two decades ago when the costs of Medicaid and, especially, Medicare began to soar wildly higher than any of the forecasts. It would be highly desirable to extend basic coverage to everybody -- but how to do it without driving the national hospital bill out of sight? Mr. Rogers' commission offers a few preliminary thoughts.

Both Boston and New Haven offer first-rate hospital care. Nearly all of it, in both cities, is in teaching hospitals, indicating well-informed doctors. There are no great differences between the two populations demographically. Why, then, do people in Boston spend twice as much per capita on hospitalization as people in New Haven?

John E. Wennberg of the Dartmouth Medical School, who made the comparison for the commission, says that hospitalization rates for major surgery are nearly identical in the two cities. But for a wide range of ailments -- for example, back trouble, gastroenteritis, pneumonia, asthma -- admission rates in Boston are far higher. Neither doctors nor patients in New Haven feel that health care there is inadequate or rationed. As far as Dr. Wennberg could discover, doctors in neither city were aware of the discrepancy in practice between them. But if Medicare patients in Boston had been hospitalized at the same rate as in New Haven, their hospital bills in the year studied, 1982, would have been $85 million rather than the $148 million that the government actually paid.

The key question, of course, is whether people's health is better in Boston than in New Haven. What is the additional hospital care producing? Nobody knows.

That's the main point that the commission wants to make. Throughout the country there are enormous variations in medical practice. Many common conditions have never been examined in the kind of rigorous clinical trials that compare the effectiveness of one treatment with another, or with none at all. It's beginning to look as though quite a lot of money might be saved in ways that have no impact on patients' health or comfort. Perhaps it might be enough money to provide for some of the 11 million children who currently have no access to medical care but through charity.