About 1 million doses of two types of childhood vaccines are being voluntarily recalled by Merck & Co. because of possible microbial contamination, and will result in a serious vaccine shortage, health officials said yesterday.

The Centers for Disease Control and Prevention and the company announced the recall late yesterday involving 11 lots of the PedvaxHIB vaccine and two lots of the COMVAX vaccine. Both provide protection against Hemophilus influenzae type B, a bacterium that can cause a serious form of pneumonia, and a condition called bacterial meningitis. Both vaccines are often popularly referred to as the Hib shot. All affected lots were manufactured after April.

Merck decided to recall the vaccines after tests at its Pennsylvania manufacturing plant revealed a malfunction in the sterilization process. Dr. Julie Gerberding, director of the CDC, described the Hib vaccine as vital to public health. Before development, there were an estimated 20,000 cases of Hib-related infections annually and 1,000 children died each year. "Now, there are fewer than 100 documented cases of Hib annually," Gerberding said.