The vaccines were manufactured in West Point, Pennsylvania, and distributed starting in April 2007. All but one lot was distributed in the United States, the company said.
"The potential for contamination of any individual vaccine is low, and, if present, the level of contamination would be low," Merck said.
Sterility tests of the vaccine lots subjected to recall did not turn up any contamination, said Merck, which added that the recall does not affect any other vaccines it manufactures.
Children who have received the affected vaccine do not need to be revaccinated because efficacy was not compromised, said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases.
She advised parents to look for signs of infection at the site of the inoculation in the days following the injection.
"After a week, you would definitely be out of the woods," Schuchat said.
Before vaccination became common, Hib struck one child out of every 200 in that age group, causing permanent brain damage in a quarter and killing 1 in 20.
The CDC has recommended that children receive Hib vaccine for nearly a decade. Fewer than 100 documented Hib cases currently are reported in the United States each year.
Health officials said they have urged Sanofi Pasteur, Sanofi-Aventis' vaccine business, to boost supplies in a bid to fill the gap caused by the recall.
The United States requires about 14 million doses annually to fully vaccinate babies and toddlers, and officials said they planned to tap vaccine stockpiles to help meet the need.
"We do expect there to be a shortage. The extent of that we're trying to find out," said the CDC's Schuchat. She added that the recall does not pose an immediate health risk because more than 90 percent of U.S. children have received the vaccine.
"We have a cushion of protection as we go into the recall," Schuchat said.
In November, Merck agreed to pay $4.85 billion to settle most claims that Vioxx, its withdrawn painkiller, caused heart attacks and strokes in thousands of users.
Shares in Whitehouse Station, New Jersey-based Merck lost 68 cents, or 1.1 percent, to close at $59.72 on the New York Stock Exchange.
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