WASHINGTON (AP) _ Childhood diseases are in full retreat in the United States because the nation has achieved the highest level of immunizations in its history, public health officials said Thursday.

Dr. Walter Orenstein, director of the national immunization program of the federal Centers for Disease Control and Prevention, said that vaccinations now are protecting more than 90 percent of U.S. children against nine diseases and about 78 percent against a 10th disease, hepatitis B.

``This is the highest coverage we have ever had,'' Orenstein said at a news conference. ``Immunization rates are at record highs and the incidence of vaccination-preventable diseases are at record lows.''

Orenstein and other experts, however, warned that all of the childhood diseases at not defeated, but merely in retreat. The diseases, such as measles, mumps and diphtheria, are still present in other countries and would rapidly infect people in the United States without a continued vaccination effort among children.

Under a new childhood immunization schedule, babies start getting shots shortly after birth and continue to get vaccines until about age 6. Catch-up shots, for vaccines that were missed, can be given as late as ages 11 to 12.

Some of the vaccines are given in combination shots, lowering the number of injections required. Children will receive 15 to 20 shots during the first six years of life, depending upon which vaccines are used. Polio, for instance, can include as many as four injections.

Dr. Samuel L. Katz of Duke University said that the immunization program has been so successful that there is a risk that some new parents will become complacent about the disease risk, leading to a breakdown in the high level of immunization that is now protecting U.S. society.

Diseases such as polio, diphtheria and rubella ``not disappeared,'' said Katz. ``They can enter the United States at any time.''

Complacency toward vaccinations led to a resurgence of measles in 1989, when the nation recorded 55,000 cases, including 150 deaths. Orenstein said this occurred because the percentage of children inoculated against measles dropped to about 67 percent.

In the 1996, the most recent year measles statistics are available, there were fewer than 500 cases nationwide and all resulted from disease brought into the United States from other countries. About 91 percent of American children are now vaccinated against measles, said Orenstein.