Measles came back with a vengeance in 2019, with cases quadrupling globally and 1,276 cases reported in the United States since the beginning of the year — the largest increase in 27 years.

Most of those cases worldwide were among people who weren't vaccinated against the preventable infection. Anti-vaccinations movements have gained ground in the industrialized nations while gaps of immunization coverage or lack of access to health care facilities plague Africa and developing nations around the globe.

But there's some good news in California. 

A new study by researchers at Stanford and the University of California, San Francisco shows the vaccination rate for measles is approaching 95% in nearly all counties of the Golden State. That auspicious number promotes herd immunity, protecting vulnerable unvaccinated people, such as newborns.

The co-authors of the study, which appears in PLOS Medicine, believe this hike in the state's vaccination rate is due to a contentious 2016 law that did away with the personal belief and religious exemptions following the 2014-2015 measles outbreak that began in Disneyland.

The new vaccine policy is associated with a 3% increase in statewide MMR (measles, mumps and rubella) vaccine coverage since the law was adopted, the researchers found, and a 2% decrease in non-medical religious and philosophical exemptions.

That jump may put the state above the critical 95% vaccinated point, which is needed for effective herd immunity against measles. "That would be very meaningful," said Stanford Health Policy's Eran Bendavid, MD, an associate professor of medicine and a co-author of the study.

The policy debates surrounding vaccine hesitancy in the United States have focused on vaccine exemptions, which provide an option for parents to waive current vaccination requirements for entry into school or daycare centers. Currently, 18 states allow nonmedical exemptions based on philosophical, personal or other beliefs.

"The factors driving vaccine hesitancy are complex and include misconceptions and misinformation about vaccine safety, low perceived risk of infectious disease, and lack of trust in health care providers," the authors write. 

The California experiment, however, could serve as an example to state legislatures and public health departments, as well as the federal government, the researchers say. 

"While we did see a small increase in medical exemptions, the much larger increase in MMR coverage suggests that the policy worked as expected," said Sindiso Nyathi, a graduate student in epidemiology, and one of the paper's first authors. "This is good news for states considering similar policies."

Sindiso said evaluating the efficacy of vaccine policies can be difficult due to lack of controls to use as comparisons, which limits the conclusions that can be drawn. To address that gap, their work used a hypothetical control group and estimated how many Californian children would have received the MMR vaccine if the law had not gone into effect. They then compared that to how many kids were vaccinated following the law's enactment in 2016.

The researchers also broke the data down by county. 

"Our county-level analysis found that greater increases in coverage were observed in counties with low coverage levels before the policy," Nyathi said. "This is good news, as it suggests that the policy was more effective in areas that had lower coverage. Similar policies may be an effective tool to bring vaccine coverage levels above herd immunity thresholds." 

While the researchers found the law work as intended, there was a small, 0.4% increase in the number of medical exemptions. 

Under the current California law, parents can request vaccination waivers for children whose medical condition might be impacted by the vaccine.

In September, Gov. Gavin Newsom signed into law another vaccination bill that will go into effect on Jan. 1. It will give the California Department of Public Health the power to revoke medical exemptions if it determines they are not medically sound. The department will also have the power to review exemptions from doctors who write more than five in one year.