"The science is clear that vaccines dramatically protect children against a number of infectious and dangerous diseases," said Gov. Jerry Brown, a Democrat, in a statement. "While it's true that no medical intervention is without risk, the evidence shows that immunization powerfully benefits and protects the community."
While all 50 states require school children to be vaccinated, 48 currently allow exemptions for families with religious objections and 20 exempt children based on parents' personal beliefs, according to the National Conference of State Legislatures.
Twelve states this year considered legislation addressing vaccine exemptions. In May, Vermont became the first state to repeal its personal belief exemption, although the law still permits exemptions for religious reasons.
Sen. Richard Pan, a pediatrician and cosponsor of the legislation, said he hopes other states will follow California's example.
"As the largest state in the country, we are sending a strong signal to the rest of the country that this can be done, that science and facts will prevail to make sound laws," Pan said.
Supporters of vaccines say the new law will protect public health.
"It is a great day for California's children," said Paul Offit, chief of infectious diseases and director of the Vaccine Education Center at Children's Hospital of Philadelphia. "Finally, someone stood up for California's children."
A growing number of parents have opted to delay or skip vaccines because of concerns over safety. Multiple studies have found vaccines to be safe, with no link to autism or other chronic conditions. Myths about vaccines continue to circulate online, however, and are promoted by a number of celebrities.
Brown noted that California children can still receive an exemption to the vaccine requirement if a physician concludes there are "circumstances, including but not limited to, family medical history, for which the physician does not recommend immunization."
Critics of vaccination had campaigned vigorously against the law, arguing that it infringed on their freedom of choice. Some have vowed to challenge the law in court. The Supreme Court has upheld state vaccine laws twice.
Santa Monica pediatrician Jay Gordon, a well-known author with concerns about vaccine safety, said some families will try to get medical exemptions.
"Many parents will be scared and angry because they have medical concerns and their doctors might not agree that the medical exemption is warranted," Gordon said. "Medical exemptions can be very legitimately granted to a large number of people with family medical histories of problems with vaccines, other medicines or with autoimmune problems. But one has to know that and have an understanding doctor."
Pediatrician Robert Sears, known for publishing an alternative vaccine schedule that delays a number of shots, predicts that many anti-vaccine parents will take their children out of school.
"When the government tries to force something on families, especially children, parents don't line their kids up to comply. They run the other way," said Sears of Capistrano Beach, Calif., who campaigned against the law. "Forcing parents to choose between vaccines and school is unnecessary and unfair. I predict that more families will shift into homeschooling and co-op, home-based educational programs over this, rather than feel coerced into doing something they don't feel is right for their child. Even more families will lose trust in the government and the medical system."
Pan said the law protects the rights of children at high risk from infectious diseases -- such as newborns or children with compromised immune systems. The law ensures that children are free to attend school without fearing for their safety, he said.
"Parents can still choose not to vaccinate their child," said Leah Russin, cofounder of Vaccinate California, which championed the law. "But if you want to send your child to school, you have to make sure that your child isn't dangerous to other children."
Studies show that strict school vaccination laws are associated with higher vaccination rates. Communities need to vaccinate at least 92% of children against measles to prevent outbreaks, said Robert Glatter, an emergency physician at Lenox Hill Hospital in New York.
In Colorado, which allows children to skip vaccines for religious or philosophical reasons, only 82% children have received both recommended doses of measles-mumps-rubella vaccine. That's the lowest rate in the USA. In Mississippi, which grants vaccine exemptions only for medical reasons, 99.7% of children have gotten both doses, according to the Centers for Disease Control and Prevention.
Research shows that children with vaccine exemptions are 35 times more likely than others to contract measles.
Many parts of the country now have large pockets of underimmunized children, which have allowed vaccine-preventable diseases to make a come back.
Nearly one in seven public and private schools have measles vaccination rates below 90%, according to a USA TODAY analysis of immunization data in 13 states.
Hundreds of thousands of students attend schools where vaccination rates have dropped precipitously low, sometimes under 50%, according to the USA TODAY analysis, published in February.
Measles killed 500 people a year in the days before vaccines. An outbreak from 1989 to 1991 sickened more than 55,000 people and killed 166, according to the CDC.