The recall is for 1.2 million doses of the vaccine for Hib, which protects against meningitis, pneumonia and other serious infections, and a combination vaccine for Hib and hepatitis B. The vaccine is recommended for all children under 5 and is usually given in a three-shot series, starting at 2 months old.

Drug maker Merck & Co., which announced the recall after testing this week showed a sterility problem in a Pennsylvania factory, said concerned parents should contact their child's doctor.

"The potential for contamination of any individual vaccine is low," said Merck spokeswoman Kelley Dougherty.

Dr. Julie Gerberding, head of the Centers for Disease Control and Prevention, echoed that in a news conference.

"This is not a health threat in the short run, but it is an inconvenience," she said.

Merck produces about half of the nation's annual supply of 14 million doses of Hib vaccine.

Barbara Kuter, executive director of pediatric medical affairs for Merck, told The Associated Press that because of the contamination, the company's production line has been shut down for at least nine months.

"Manufacture of vaccines is pretty complicated, and we have to basically make some changes in the process," then get approval from the Food and Drug Administration before resuming production and shipments, Kuter said. Merck hopes to restart production in the fourth quarter of 2008, she said.

"It's likely that there's going to be a shortage of this product," Kuter said, adding that the impact on the public is unclear because the other company making the vaccine in the U.S., Sanofi Pasteur, may be able to produce more.

Health officials said they already are talking about prioritizing shots for American Indian and Alaska Native children, who are considered at higher risk for Hib-caused illnesses, said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases.

Health officials said they did not know how many of the 1.2 million doses were administered to children.

The recalled doses are considered potent, so children who got vaccine from the recalled lots will not have to be revaccinated, Schuchat said.

Parents will probably be concerned, CDC officials acknowledged. Should the vaccine later prove contaminated, health officials believe most children will experience, at worst, a skin irritation around the vaccination site. Problems could be worse for children with compromised immune systems.

Such problems would have appeared within one week of the vaccination, Schuchat said, adding that there have been no reports suggesting vaccine contamination so far.

The contamination involved unspecified equipment used in making the vaccine, which involves taking concentrated Hib virus, diluting it and combining it with other agents. Kuter said that during a routine evaluation of Merck's West Point, Pa., vaccine plant, a sterility test determined that the equipment was contaminated with a bacteria called Bacillus cereus, or B. cereus.

It is a spore-making microorganism commonly associated with food poisoning and has caused diarrhea and vomiting in people who eat contaminated foods.

"It's one of the most common organisms" around, Kuter said.

The recall is likely to heighten a debate over childhood vaccines and their safety and whether too many are required. Some parents are distrustful and suspect some vaccines of being linked to autism, although scientific studies have not shown such a connection.

This week, New Jersey took a controversial step toward becoming the first state to require flu shots for preschoolers after a health advisory board backed new vaccine requirements over opposition from parents.

Merck, based in Whitehouse Station, N.J., is one of the few drug makers that produces a significant number of vaccines.

While the company took a black eye with its September 2004 withdrawal of the painkiller Vioxx due to increased risk of heart attacks and strokes, the company has been performing well recently. On Tuesday, it gave an upbeat assessment in its annual briefing for analysts.

Five weeks ago, Merck reached a deal to settle up to 50,000 Vioxx lawsuits for $4.85 billion, an amount expected to save the company millions in trial costs.

Its stock price has more than recovered from its post-Vioxx slump, a two-year-old restructuring plan is going well and profits are up. For example, Merck posted a 62 percent increase in its third-quarter profit as revenues jumped by double digits.

The company also has had an impressive seven new products approved for U.S. sale in the last two years, including three vaccines: RotaTeq, to prevent an intestinal virus that is the top cause of early childhood diarrhea; Zostavax to prevent shingles, and Gardasil, to block the virus that causes cervical cancer.