The Centers for Disease Control and Prevention said Thursday that inventories of chicken pox and measles-mumps-rubella vaccines are more than 1 million doses below where they should be. The shortfall is equal to 18 percent of the chicken pox vaccine used annually, and 8 percent of the MMR vaccine.

The CDC is asking doctors to postpone indefinitely the second MMR shot, usually given between 4 and 6 years of age. Doctors also are urged to delay giving varicella -- chicken pox -- vaccine until children are 18 months, rather than between 12 and 18 months.

The postponements are the latest requested by the CDC over two years, all stemming from the same problem: Eight of the 11 recommended childhood vaccines, plus some adult vaccines such as the flu shot, are in short supply.

"We have been in a shortage situation for almost a year and a half now with tetanus-containing vaccines and with the new pneumococcal vaccine for children," said Dr. Jane Seward at the CDC. "But this is the first time we have been short for varicella and MMR."

The latest shortfall is the result of a one-month manufacturing interruption last year at Merck & Co. after a Food and Drug Administration inspection. Merck is the only U.S. maker of varicella and MMR vaccines.