But Fisher said that assessment was based on anecdotal evidence. "I don't know how common it is," she said.
Her website reports six cases of adverse reactions to the vaccine, none of which include mental retardation. But, "to suggest that it does not happen is inaccurate," she said.
Siegel does not disagree that brain inflammation can occur, but he said it is rare. "Encephalitis (swelling of the brain) is a remote and rare side effect of many vaccines, including the flu vaccine," he said. "It is more common in live virus vaccines, which HPV is not."
A recent Institute of Medicine report, sponsored by the Department of Health and Human Services, also said severe reactions were rare.
In the United States, two HPV vaccines are available, Cervarix, made by GlaxoSmithKline, and Gardasil, made by Merck & Co.
For more information on HPV vaccine, visit the U.S. Centers for Disease Control and Prevention.
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