People who oppose repealing the personal belief exemption gathered outside California's Capitol in Sacramento on Wednesday. Pauline Bartolone/Capital Public Radio hide caption

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Leah Russin, of Palo Alto, Calif., holds her son, Leo, 16 months, as she speaks Wednesday at a news conference in support of proposed state legislation that would require parents to vaccinate all school children. Rich Pedroncelli/AP hide caption

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A mom at the Community Clinic in Louisiana Township, about 15 miles from Monrovia, says all of her children have been vaccinated. Jon Hamilton/NPR hide caption

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Daniela Chavarriaga holds her daughter Emma as Dr. Jose Rosa-Olivares administers a measles vaccination at Miami Children's Hospital. Joe Raedle/Getty Images hide caption

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On the outskirts of Islamabad, a Pakistani health worker vaccinates an Afghan refugee against polio. Muhammed Muheisen/AP hide caption

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