ASHINGTON, Dec. 1 — Federal and state officials and doctors across the country are expressing alarm at shortages, delays and uncertainty in the production and supply of major childhood vaccines.

The delays and shortages have forced states to ration vaccines, set new priorities and revise their immunization policies and requirements. Colorado, for example, has reduced the number of shots a child must have to enroll in school or in a day care center, and as a result, some children have less protection against diseases like diphtheria and whooping cough.

Dr. David R. Johnson, chief medical executive in the Michigan Department of Community Health, said, "We don't know from month to month, week to week or day to day what will happen to our vaccine supply.

In a bulletin sent to states this week, the federal government reported shortages of vaccines against 4 of the 11 diseases preventable through routine vaccination of children: diphtheria, tetanus, whooping cough and pneumococcal disease, which can cause meningitis and pneumonia. In addition, the government said, many doctors have experienced serious delays in the delivery of vaccine for six other diseases: influenza, chicken pox, measles, mumps, rubella and hepatitis B.

Two of the largest manufacturers, Merck & Company and Wyeth Lederle Vaccines, a unit of American Home Products Corporation, acknowledged the problems. Merck said it was "experiencing intermittent shortages of some of our vaccines," including the one for measles, mumps and rubella. Merck said that it curtailed vaccine production over the summer because it was making changes at its manufacturing plant in West Point, Pa.

In a letter to doctors, Wyeth recently apologized for the inconvenience caused by shortages of Prevnar, its vaccine for pneumococcal disease. Douglas Petkus, a spokesman for Wyeth, said demand had exceeded expectations, and the company had to change manufacturing methods at a plant where it experienced a bottleneck.

The federal government estimates that the nation needs 1.4 million to 1.6 million doses of Prevnar each month to protect infants and toddlers against life-threatening diseases caused by the pneumococcal bacteria. But, it says, the supply averaged only 707,000 doses a month in September and October. Children are supposed to get four doses in the first 15 months of life.

Some manufacturers have halted production of vaccines after concluding that they were unprofitable. Drug companies are not required to inform the government they intend to stop making a vaccine.

Lawsuits used to be a major problem for manufacturers, but the level of concern has dropped sharply since 1986, when Congress created a no- fault system to compensate people injured by childhood vaccines. Under the program, the government reviews and pays claims for injuries, using money derived from an excise tax on every dose of vaccine.

Now, members of Congress are asking whether the federal government ought to play a larger role in managing the supply of vaccine. The Institute of Medicine, an arm of the National Academy of Sciences, said last month that the government should create a National Vaccine Authority to help companies produce and distribute vaccine.

Senator Jack Reed, Democrat of Rhode Island, said the idea was worth considering because "the vaccine shortages pose a real threat to our public health system."

Dr. Bruce A. Weiss, chief medical officer at AvMed Health Plan, a nonprofit health maintenance organization in Florida, said the shortages could undermine years of work.

Two of the four companies that made vaccine against diphtheria, tetanus and whooping cough — Wyeth and North American Vaccine, now part of Baxter International — have dropped out of that market.

Federal health officials said the shortages of this vaccine might continue until late next spring.