The Food and Drug Administration may crack down on states that don't start requiring more schoolchildren to get vaccinated.

Officials says the nationwide measles outbreak is most prevalent in areas with high concentrations of unvaccinated children.

Dr. John Zaso, a pediatrician, says this wouldn't be the first time federal authorities force immunization. It has happened when a disease reached epidemic proportions, causing a public health crisis. He says those include smallpox and polio outbreaks in the 1940s and '50s.

Currently, 47 states allow parents to opt out of childhood vaccines for religious reasons. Of those, 17 states allow parents to opt out because they feel that vaccines violate their personal or philosophical beliefs.

Some of those parents made their voices heard on a Facebook page called  "Stop Vaccinations Now." They say they're concerned vaccines may cause neurological problems and that "people should have a choice" when it comes to vaccinating their kids.

Jennifer Oddo, of West Islip, says vaccination should be a parent's choice, but not when there's a public health crisis involving measles or other diseases.

The FDA has not yet announced any firm plan about making vaccinations mandatory.

The feds would make a decision after consulting with the Centers for Disease Control and Prevention, as well as local and state health departments where an outbreak has occurred.