Unvaccinated Clusters Leave Communities Vulnerable to Illness, Research Shows

— -- A study of vaccination rates among children in northern California showed that there are "clusters" where parents aren't vaccinating their children -- and they're at risk for catching preventable diseases.

Researchers at Kaiser Permanente studied records of more than 150,000 children in the Bay Area from birth to age 3, and identified five clusters where children were under-immunized, meaning their parents either refused to vaccinate them or they missed one or more vaccines. The percentage of under-immunized children in these often well-educated clusters was between 18 percent and 23 percent. Outside them, only 11 percent of the children were under-immunized.

ABC News Chief Health and Medical Editor Dr. Richard Besser said not only does this put the unvaccinated children at risk for getting illnesses such as whooping cough, but it puts those around them at risk because vaccines aren't "100 percent protective." The measles vaccine, for instance, is 95 percent protective.

"The more child you're around who didn't get vaccinated, the more likely you are to be exposed to that and get the disease even if you were vaccinated," he said today on ABC News' "Good Morning America."

Host Lara Spencer asked him what he would say to parents who think there are "just too many vaccines."

"As a pediatrician, I've seen so many of these diseases cause suffering in children," Besser said. "And every time we get a new vaccine, I think it's a wonderful thing."

He stressed the importance of getting vaccines on time and using online tools to find out vaccination rates in their schools. If they can send their child to a school with higher vaccination rates, they should, he said.