The increasing number of kindergartners entering school without immunizations poses a risk to others, especially children who have legitimate medical exemptions that prevent them from getting their shots, said Linda Davis-Alldritt, a school nurse consultant at the California Department of Education.
"Disease prevention is really a very important thing," she said. "These are diseases that can be very serious, and it can cause death and it can cause long lasting illnesses."
Parents receive information from schools and the Department of Education about the importance of inoculation and the dangers of unvaccinated children spreading infectious diseases to the rest of the community.
Overall immunity of a population to illness typically is achieved when 90 percent of the population is properly immunized. But Talarico, California's top immunization officer, said that can be misleading because unvaccinated children tend to cluster in pockets where like-minded parents decide to forgo immunizations.
"When we see these clusters, that represents the possibility of transmission of disease more quickly and in a more sustained fashion," he said.
A cluster of unimmunized children in San Diego led to an outbreak of measles in 2008 that infected 12 children. Nine of them had not been inoculated because of their parents' objections, while three others were too young to be immunized.
In some schools, as many as 30 percent of kindergartners are vulnerable to at least one vaccine-preventable communicable disease, according to data from the state Department of Public Health. The majority of schools have 100 percent immunization rates.
Certain areas of California have higher exemption rates than others. In Marin, Sonoma and Santa Cruz counties, more than 6 percent of incoming kindergartners in 2010 had parents file a personal belief exemption. By comparison, Los Angeles, San Francisco and Fresno counties had rates below 2 percent.
Barbara Lowe Fisher, co-founder and president of the National Vaccine Information Center in Vienna, Va., said parents sometimes decide to file the personal belief exemption form because they cannot find a doctor who will sign a medical exemption form.
"We've had families whose children have had reactions to vaccines, and some of them have become injured or even died. They want to make an independent, informed decision for their other children, and they can't find doctors who will write medical exemptions," she said.
She said the mission of her nonprofit is to prevent vaccine-related injuries or death through public education. It does not take a position on whether parents should have their children vaccinated, but it does defend a parent's right to opt out of a vaccine.
Some parents base their decision on what they hear from others or see on the Internet, rather than in consultation with a medical expert, said Catherine Flores Martin, director at the California Immunization Coalition, a Sacramento nonprofit that receives some of its funding from vaccine manufacturers.
"Parents have access to so much information on the Internet and I think a parent, even when they're really well educated, will have a hard time sifting through the credible resources versus the anecdotal stories," she said. "You can find whatever you want on the Internet to support your belief."
Tayefe Mohajer reported from Los Angeles.