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A new study looks at incoming kindergartners in California, and finds that the lack of vaccination is threatening herd immunity in some schools, and that some measures of risk have doubled in just three years.
California, like other states, has a mandatory immunization schedule, set as a requirement for children entering school. But California is also one of 20 states that allows a personal belief exemption, where parents can file notice that they have a personal issue with vaccines, and get their kids into schools despite a lack of vaccination. The rates of people asking for these exemptions has been slowly climbing, rising from half a percent in 1996 to 1.5 percent in 1997.