More than 1 in 10 American parents refuse or delay vaccinating their children, mainly because of safety concerns, according to a national survey.

The results suggest more than 2 million infants and children may not be fully protected from diseases like measles and whooping cough spreading in schools and communities.

The vaccines that we recommend have been so effective in largely eliminating the vaccine-preventable diseases that most parents don't have first, second or even third-hand experience with these diseases, said Dr. Amanda Dempsey, one of the authors of a new report based on the survey from the University of Michigan in Ann Arbor.

According to the researchers, parents who skip or delay vaccines typically cite safety concerns. Parents were most likely to skip vaccination against H1N1 (swine flu) and seasonal flu, the study says. Parents were least likely to skip the polio vaccine.

The Internet survey included 748 parents of children six months to six years old. Of those, 13 percent said they used some type of vaccination schedule that differed from the CDC recommendations.

The results were released online in the journal Pediatrics.

The Centers for Disease Control and Preventions recommends vaccinations for MMR (measles, mumps and rubella), whooping cough, chicken pox, hepatitis and seasonal flu, among other diseases.