Rotarix, made by GlaxoSmithKline, was approved by the FDA in 2008. The contaminant material is DNA from porcine circovirus 1, a virus from pigs that is not known to cause disease in humans or animals, Hamburg said.

About 1 million children in the United States and about 30 million worldwide have gotten Rotarix vaccine, she said.

The FDA learned about the contamination after an academic research team using a novel technique to look for viruses in a range of vaccines found the material in GlaxoSmithKline's product and told the company, Hamburg said. The drug maker confirmed its presence in both the cell bank and the seed from which the vaccine is derived, suggesting its presence from the early stages of vaccine development, she said. The FDA then confirmed the drug maker's findings.

In the next four to six weeks, the drug agency will convene an advisory committee to make recommendations and seek input on the use of new techniques for identifying viruses in vaccine, Hamburg said.

"We're not pulling it from the market, we're just suspending its use during this period while we're collecting more information," she said. "It should not be in this vaccine product and we want to understand how it got there. It's not an easy call and we spent many long hours debating the pros and cons but, because we have an alternative product and because the background rates of this disease are not so severe in this country, we felt that the judicious thing to do was to take a pause, to really ask the critical questions about what this material was doing in the vaccine, how it got there."

The research group that discovered the contamination has asked not to be identified pending its paper's publication in a scientific journal, Hamburg said.

• Rotavirus Vaccine Developed By Glaxo
  Linked To Convulsions & Pneumonia