Merck & Co. Inc. announced yesterday that it was voluntarily withdrawing 1.2 million doses of a common childhood vaccine because of fears that it might be contaminated with bacteria.

Federal officials stressed that the recall was precautionary and that they had not found any cases of contaminated vaccine. Nor have they found any unusual cases of infection from the disease the vaccine stops, Haemophilus influenzae type b or HIB, which can cause meningitis and pneumonia.

But the company did find contamination on equipment in October, resulting in the voluntary recall yesterday. Experts now are bracing for a shortage of vaccine in the months ahead.

"I want to emphasize this is not a health-threatening situation in the short term," said Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention, in a conference call with reporters. But "this is going to be a great inconvenience."

The disease, which is not related to the flu, used to be a great killer of children, infecting 20,000 a year and killing 1,000, Gerberding said. But the HIB vaccine became mandatory in 1990 for children, who now get it starting at age two months. "There are fewer than 100 HIB cases each year," Gerberding said.

Still, the vaccine is difficult and complex to make, and only two companies still produce it. Gerberding said that a shortage would exist and that the government might have to issue priorities in a few days clarifying who should get the vaccine first.

"There will be children who won't get the HIB vaccine who may be hurt by that," said Paul A. Offit, a vaccine expert at Children's Hospital of Philadelphia. "They don't know who they are yet."

John F. Domzalski, acting Philadelphia health commissioner, said about 4,000 children in the city potentially received the vaccine. He said health workers would check lot numbers and alert doctors.

Many pediatricians already have the vaccine in their offices, and are being asked to return the products, known as Pedvaxhib and Comvax, a combination that includes Pedvaxhib and Hepatitis B vaccine. Merck is taking returns and compensating providers through its national service center at 1-800-672-6372.

"This is absolutely the worst time for something like this," said Daniel R. Taylor, a pediatrician at St. Christopher's Hospital and an assistant professor at Drexel University College of Medicine. It is already high season for the sniffles and other illnesses, he said. And Taylor expects a barrage of calls from parents, who typically do not know the manufacturer, let alone the lot number, of the vaccine their child received.

Parents should know that a child can show signs of infection within days of receiving the vaccine, and that it typically begins as skin bumps or inflammation at the needle site, experts said.

But if nothing appears within a week, the child will not be affected, said Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases.

Children who were vaccinated weeks or months ago are also protected, she said, and will not need revaccination.

Parents can report any suspected infections through the federal government's VAERS vaccine reporting system at 1-800-822-7967.

Federal officials said about 14 million doses were needed to fully vaccinate U.S. children against the bacteria. The vaccine, which is given year-round when needed, is made by two firms, Merck and Sanofi-Aventis, a vaccine-maker with facilities in Swiftwater, Pa., in the Poconos. Merck has its vaccine operations in West Point, Montgomery County. Each company controls about 50 percent of the market.

Federal officials said Sanofi would increase its production, and the CDC will also look at releasing some of the three-quarters of a million doses the agency has stockpiled.

Experts still expect a shortage. But they do not expect any disease outbreaks. Gerberding said in other parts of the world where vaccines have been short, "it's taken more than a year to have any noticeable effect on disease rates."

Still, the announcement represents a big setback for Merck, which recently reported strong financial results in large part because of its robust vaccine business, which employs more than 10,000 people in West Point.

The recall affects 10 lots of Pedvaxhib and two lots of Comvax, manufactured since July 2006 and distributed starting April 2007.

The vaccine, which is given in three doses, is recommended for all children starting at 2 months and up to their fifth birthday. It costs the government $10.83 while the list price is $22.77 per dose.

Merck said it found contamination on some equipment during a routine inspection in October. This month, it traced the cause to a process improvement made in mid-2006.

Contact staff writer Karl Stark at 215-854-5363 or kstark@phillynews.com.

Inquirer staff writer John Sullivan contributed to this article.