Vaccination rates among California kindergartners rose last year, and fewer students skipped shots because of their parents' personal beliefs about immunizations, according to state officials. The changes, though slight, suggest that a state law barring parents from refusing to have their kids vaccinated appears to have had some impact even before taking effect later this year.

More than half a million children attend public or private kindergarten in California. Information collected during the fall for the 2015-2016 school year shows 92.9 percent of children received all of the required vaccinations. That's up from 90.4 percent in 2014 and 90.2 percent in 2013, according to a report by the California Department of Public Health.

The information tracks the vaccination rates for polio; diptheria, tetanus, and pertussis (DTAP); measles, mumps, and rubella (MMR); hepatitis B; and varicella, better known as chicken pox. The data show just 2.38 percent of children entering kindergarten had a personal belief exemption, which parents can use to skirt vaccination requirements. That's slightly fewer than in 2014, when 2.54 percent of children had the exemptions, and 2013, when 3.15 percent had them.

For years West Virginia and Mississippi have barred personal belief exemptions and required nearly all children attending school to be vaccinated; more recently a number of other states have joined their ranks. Last year California state lawmakers, prompted by a measles outbreak that started at Disneyland and eventually sickened 113 people, passed SB277, eliminating parents' ability to claim that their personal beliefs prohibit them from vaccinating their children. Governor Jerry Brown signed the bill into law on June 30 last year. And although schools don't need to implement the law until July 1 this year, California is already seeing a decline in exemptions.

Transmission of measles is largely, but not entirely, prevented by vaccines like the MMR, a shot administered in two doses—the first around age 1 and the second between 4 and 6. Nationwide, the number of measles cases dropped so dramatically after the vaccine's introduction in 1963 that in 2000 the Centers for Disease Control and Prevention declared the disease eliminated in the United States. But the disease has seen a re-emergence in recent years, due in part to a medical journal article, which was subsequently retracted, that falsely linked autism with the MMR. The disease claimed its first fatality in 12 years last year, killing a woman in Washington. Nearly half of the people who contracted measles in the US in 2015 were not vaccinated, according to the CDC.

Because SB277 does not take effect for another five months, its impact on childhood vaccination rates won't be known until data for the 2016-2017 school year is released next year. Still, the upward trend is promising. Los Angeles county, which has the state's largest school district and is responsible for educating one-quarter of all California kindergarteners, saw a four-point rise in its vaccination rate, an increase of nearly 9,000 children. The rate among San Francisco kindergartners climbed from 86.4 percent during the last school year to 92.5 percent for the current one.

That said, more than 13,000 children starting kindergarten in California in 2015 were not vaccinated due to a personal belief exemption. More troubling, half of the state's 58 counties reported that their measles vaccination rate for kindergarteners did not meet the threshold for herd immunity—that is, the level at which transmission of the disease can be most effectively prevented. Trinity county, in northern California, claims the lowest vaccination rate, with just 77.0 percent of enrolled kindergarteners up-to-date on their shots; Nevada county, northwest of Lake Tahoe, is right behind Trinity with 77.1 percent of kids immunized. The award for the county with the highest vaccination rate goes to Alpine, south of Lake Tahoe, though the county reported data for a mere six kindergarteners.

The state's data isn't foolproof, however. When WIRED reported last year that some day care facilities affiliated with prominent Silicon Valley companies had subpar vaccination rates, some of them claimed their rates were higher than the state's data suggested. There is some evidence to support this. For example, the state reported that 83 percent of the 18 kindergartners enrolled at the San Francisco campus of AltSchool, a chain of private schools founded by former Google executive Max Ventilla, were vaccinated. The remaining 17 percent claimed a personal belief exemption. Yet the school says 83 kindergarteners matriculated in San Francisco this year.

"We do have a small percentage of students who had existing medical exemptions or personal belief exemptions," says Michael Ginty, head of safety and security at the school. "But we have more than 18 kindergarteners, so this information doesn't seem accurate to me. In the data we reported at the end of October, of all the kindergarteners and seventh graders, we had only one student with a personal belief exemption. All other students were immunized or on track to be immunized."

But Carlos Villatoro of the California Department of Public Health says the state stands by the numbers. "AltSchool in San Francisco reported data individually on three schools, each with separate addresses and identification numbers," says Villatoro. The DPH data refers to one of the school's three San Francisco locations. "That 83 percent in the spreadsheet refers to 15 out of 18 children who were reported as having received all required immunizations."

Some of the companies identified WIRED's report last year promised this year's numbers would be better. That data will be available in February.

UPDATE 01/21/16, 3 pm PST: This story was updated to include a comment from the California Department of Public Health.