How safe is Social Security? Very safe, according to the sober judgment of its trustees. The trustees' annual report shows that the retirement and disability programs will stay in good shape well into the next century even if the economy is poor. And the Medicare program, previously thought to be on the brink of disaster, now looks as if it will remain healthy until near the end of this century.

After all the prophecies of gloom and doom from left and right, this good news may come as a surprise. But remember that Congress has made important reforms in these programs in recent years. As a result, the retirement fund is now building a small surplus while repaying money borrowed earlier from the Medicare and disability funds. Over the next few decades, those surpluses are expected to become enormous. This raises the interesting question of whether it makes sense to build up huge government reserves -- though they will be needed to pay future benefits -- and, if so, how best to invest them.

Medicare's future is not so rosy. Burgeoning medical technology and increasing longevity mean that medical costs, especially for the elderly and disabled, are likely to outpace general economic growth for the foreseeable future. But experience under the administration's tough new hospital payment system suggests that outflow from the hospital trust fund won't exceed payroll tax receipts and reserves until 1998 -- a few years ago insolvency was expected by 1987. Two qualifications, however, deserve note.

The first is the assumption that Congress will go along with the administration's proposals to continue keeping a tight lid on hospital costs. The second is that the hospital system can continue to stand the strain of the new controls without serious consequences for patients. No doubt there are many ways that hospitals and doctors can economize while still providing high-quality care. As both the government and private employers have begun to put pressure on insurers to lower costs, new competition among hospitals and other providers has helped restrain the runaway growth of health-care costs.

But there are also indications that some of these savings are being produced at too high a social cost. The Wall Street Journal, for example, recently reported that sharp cutbacks in nursing and support staff in many hospitals have put such strain on nurses that many are near collapse while others have been forced to skimp on the quality of patient care. That's a situation that will bear close watching. Anyone who has ever been in a hospital knows the importance of good nursing. It would be a severe mistake to ensure Medicare's future solvency by taking advantage of the hospital system's most undervalued asset.