The Centers for Disease Control and Prevention urged Americans to vaccinate their children against measles as the number of people hit by a new outbreak of the disease climbed past 100.

CDC Director Dr. Thomas Frieden said in an interview with CBS’s Face the Nation on Sunday that he was “very concerned” about the possibility of a large outbreak and said the best way to prevent the disease was with vaccinations he called “safe and effective.” He blamed a lack of vaccinations for the resurgence of a disease that the CDC said was eliminated in the U.S. in 2000.

“What we’ve seen is, as over the last few years, a small but growing number of people have not been vaccinated. That number is building up among young adults in society, and that makes us vulnerable,” Frieden said. “We have to make sure that measles doesn’t get a foothold in the U.S. It’s been actually eliminated from this country for 15 years. All of our cases result, ultimately, from individuals who have traveled and brought it back here.”

The CDC said that 102 people from 14 states have been reported to have measles, and that most of those cases were tied to a December outbreak at Disneyland or Disney California Adventure Park in Anaheim, Calif.

The vaccination rate for measles in the U.S. is 92 percent. Some parents don’t vaccinate their children because they are unaware of the threat posed by measles, Frieden told Face the Nation.

But some parents are part of an anti-vaccination movement that believes in a link between vaccines and autism. The movement is fueled in part by a 1998 study by Andrew Wakefield in the British medical journal The Lancet — even though The Lancet retracted the study in 2010.

Additionally, in 2010, British medical officials took away Wakefield’s medical license for alleged ethical lapses, including conducting invasive and unnecessary medical procedures on children.

Wakefield stands by his study.

FRONTLINE’s The Vaccine War (2011) explored the bitter debate between the public health establishment and a growing movement of parents who remain fearful of vaccines despite established scientific consensus about their safety. You can watch it here: