Amid the measles outbreak traced to Disneyland visitors nearly a year ago, criticism was directed at California parents who got vaccine exemptions for their children based on "personal beliefs," or nonmedical reasons.

A new study finds a bit of good news: A fair number of exempted children in the state may have been vaccinated. The lead author of the study, published in the journal Vaccine, was Alison M. Buttenheim, an assistant professor at the University of Pennsylvania School of Nursing.

The study looked at kindergartners from 2009, when California collected additional data on children whose parents sought exemptions.

In a separate blog post, Buttenheim listed reasons some of these exempted children may have been vaccinated. Among them: Parents may have requested exemptions simply because they were unable to find their children's vaccination records.

Along with the good news comes a note of caution, the authors wrote. Previous studies have calculated unvaccinated children's risk of getting measles, but they relied on data from exemption rates, not actual vaccination. Because some of those children are, in fact, vaccinated, that suggests the true risk of not being vaccinated is even higher.

- Tom Avril