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According to a new study from the American Academy of Pediatrics, a growing number of parents believe vaccinations are unnecessary.

The AAP surveyed more than 600 pediatricians in 2006 and then again in 2013, asking them to estimate the percentage of parents they encountered who wished to refuse or delay vaccines and the parents rational for doing so.

By 2013, nearly 9 out of 10 pediatricians say they were asked by at least one parent in their practice to altar their child's immunization schedule.  Pediatricians in the study responded that 73 percent of parents refusing or delaying vaccines for their children in 2013 were doing so because the parents believed the vaccines were unnecessary.

Vaccinations have prevented an estimated 322 million illnesses, 21 million hospitalizations and 732,000 deaths for Americans born between 1994 and 2013, according to the U.S. Centers for Disease Control and Prevention.  Here's more from abcnews.go.com.