An increasing number of parents in the United States are citing faith to avoid getting their children vaccinated, according to a new study -- even though no major religion opposes vaccination.

Childhood shots have been proven safe and effective, and prevent millions of deaths each year. Just 15 states allow parents to opt out of them for philosophical or personal reasons. But new research suggests that anti-vaxxers may be finding a workaround: exploiting religious belief exemptions that are permitted in 45 states and the District of Columbia.

Published Monday in the medical journal Pediatrics, the study analyzed data from the US Centers for Disease Control and Prevention and found that religious exemptions have been increasing since 2011 -- and were significantly more likely in 2017 than six years prior.

Parents were more likely to claim religious exemptions for vaccines in states that didn't permit personal belief exemptions, according to the research. That suggests a "replacement effect," according to Dr. Joshua Williams, the study's lead author and an assistant professor of pediatrics at the University of Colorado.

"When you give parents two options in a state, personal belief and religious exemption, a very small percent of parents are actually opting for religious exemptions if given an alternative," he said.