(CNN)There are now five states in the United States that do not offer personal, philosophical or religious exemptions from school vaccination requirements: California, Mississippi, West Virginia, Maine and New York.
The idea behind these controversial state laws -- which have received opposition from anti-vaccination activists -- is that requiring more children to get vaccinated, unless they have a medical reason not to, can help reduce the spread of disease.
Yet does such strict legislation really lead to more children being up-to-date on their vaccine requirements? A new study suggests yes, at least in California.
The new study involved analyzing annual data about kindergartners and their vaccination status at the start of school between 2000 and 2017. They looked at the data before and after three specific interventions took place: two vaccine-related policies and one education campaign.
The yearly rate of kindergartners without an up-to-date vaccination status rose from 7.8% in the year 2000 to 9.84% in 2013 -- before the policies and campaign -- and then the rate fell to 4.87% in 2017 -- after the policies and campaign -- according to the study published in the medical journal JAMA on Tuesday.
In 2014, when California did allow personal exemptions, a bill passed that required parents in the state to submit proof they had discussed the risks of not vaccinating their children with a health care practitioner prior to obtaining a personal belief exemption.
Then in 2015, the California Department of Public Health and local health departments launched a campaign to educate school staff on the proper application of admission criteria, giving students more time to catch up on vaccinations. In 2016, California implemented a bill banning all personal belief exemptions. A Disneyland-linked measles outbreak that hit California around that time was cited as the catalyst for the legislation.
"We noted a big decrease in the likelihood that a not-up-to-date kindergartener would have contact with another not-up-to-date kindergartner within their school," the study's first author Cassandra Pingali, a fellow at the US Centers for Disease Control and Prevention's Immunization Services Division in Atlanta, said in an email.
CDC was not involved in the study, as Pingali conducted the research while she was a graduate student at Emory University.
"These results indicate that a vaccine preventable disease outbreak is less likely to occur, and the interventions were effective at increasing vaccination rates in California kindergartners," she said.
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The study included publicly available data on kindergarten enrollment and vaccination status from the California Department of Public Health between 2000 and 2017. During that time period, about 9.3 million children started attending kindergarten across more than 10,000 California schools, according to the study.
The study had some limitations, including that only schools with 10 or more students were included in the data, the analysis only involved the time between 2000 and 2017, and more research is needed to determine whether similar findings would emerge in other states. Two of the study authors also reported financial ties to Pfizer and other companies.
The study itself was funded by the National Institutes of Health.
All in all, "health policy does not occur in a vacuum, and California was responding rapidly to address disease outbreaks in the state," Pingali said in the email.
The study suggests that legislative activity can make a real-world impact, said Dr. Peter Hotez, who was not involved in the study but is a vaccine researcher and dean of the national school of tropical medicine at the Baylor College of Medicine in Houston. He is also the author of a book about his daughter, "Vaccines did not Cause Rachel's Autism."
"What they haven't done is they haven't looked past 2017 in this study," Hotez said. That data would be valuable as other US states might also begin closing vaccine exemptions to prevent the return of measles.
So, he said, in California, "with those legislative interventions there was an immediate and dramatic impact. The question then becomes: Is this sustainable?"
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Dr. Matthew Davis and Seema Shah, both affiliated with Northwestern University, co-authored an editorial that published in JAMA alongside the new study on Tuesday.
"Although the study did not measure actual outbreaks of disease, reductions in children's risk of contracting measles are a promising outcome in California resulting from policy changes," Davis and Shah wrote in the editorial.
They added that the study findings "suggest that adopting this policy nationwide could substantively limit outbreaks of vaccine-preventable diseases such as measles."
