All states allow medical exemptions, and most permit religious exemptions. However, an increasing number of the vaccine exemptions fall into another category -- parents objecting to inoculations because of personal beliefs, usually tied to an unproven premise that vaccines are linked to autism and other disorders, The New York Times reported.

Twenty states allow some type of personal exemption, Johns Hopkins University reported.

"I saw medical studies, not given to use by the mainstream media, connecting them with neurological disorders, asthma and immunology," said Sybil Carlson, of San Diego, whose 6-year-old son is immunized against some diseases but not measles.

She said she knew she was putting other children at risk.

In 1991, less than 1 percent of children in personal-belief exemption states weren't vaccinated based on the exemption, said Saad Omer, an assistant scientist at the Johns Hopkins Bloomberg School of Public Health. By 2004, the figure rose to 2.54 percent.

"If you have clusters of exemptions, you increase the risk of exposing everyone in the community," Omer said.