Researchers at UMass Amherst say it's unclear whether requiring vaccines in schools directly increases the number of children who get them.
Data collected from 50 years of studies do show vaccination rates are higher when schools have vaccine mandates.
But UMass professor Devon Greyson said that could be for a variety of reasons. There may be higher vaccine awareness in those places, or better insurance coverage.
"We tend to think of these mandates as convincing parents," Greyson said. "However, mandates also have other effects on the whole vaccination system."
One surprise, she said, was that requiring children to get the relatively new HPV vaccine actually brought down the rate in places with high vaccine skepticism.
Greyson said policymakers considering new or stronger mandates should first consider why people in those areas don't vaccinate.
A policy that works well in one area could backfire in another.