ATLANTA -- Merck recalled about a million doses of a childhood vaccine. The move was made after testing showed a sterilization problem at its plant in Pennsylvania.

Merck is not aware of any harm to children who received the vaccine -- known as HIB -- which prevents meningitis, pneumonia and certain severe infections. The vaccine is a three-dose shot recommended for all children under 5 and is usually given to infants starting at two months old.

Merck says the vaccine itself doesn’t appear to be contaminated but said it will be in short supply while the company works on its sterilization problem.

The Centers for Disease Control says this is not a health threat and has received no reports of adverse events.

However, this is not a “no risk” situation. There is a small chance that unvaccinated children could develop these illnesses but experts say, because 94 percent of toddlers have already been vaccinated, the illnesses should not be circulating in the United States.

Officials plan to meet to analyze the vaccine supply and determine whether changes need to be made to vaccine recommendations. Merck normally makes about half the supply of this vaccine. Sanofi Pastuer, another manufacturer, could increase its production.

Since HIB vaccinations started, annual cases have gone down from 20,000 to 100 per year.