All this adds up to an ethical quagmire, say the researchers.

"Choosing not to vaccinate a child puts others at risk," said study co-author Arthur L. Caplan, PhD, Drs. William F. and Virginia Connolly Mitty Professor of Bioethics, Department of Population Health, Division of Medical Ethics, NYU Langone Medical Center. "Though making the opt-out process more difficult may reduce the numbers of exemptions granted, it is also ethically problematic." According to Dr. Caplan, this conflict poses a challenge for public health policy makers and continues to incite social debate.

All 50 states and the District of Columbia allow vaccination exemptions for medical reasons; 49 states and the District of Columbia allow exemptions for religious reasons; and 18 states allow exemptions for school-aged children for philosophical reasons (people who strongly object to immunization for reasons not associated with their religious beliefs). For the purposes of the recent study, researchers examined the process that certain states require parents to follow to obtain non-medical exemptions for school-age children, which are more prevalent than medical exceptions: About 80 percent of all vaccination exemptions in the 2011-2012 school year were non-medical.

Researchers also studied the complexity of individual states exemption processes to determine if they played a role in parents' decisions to opt-out. "Easy states" require parents to complete an easily obtained standardized form; "medium states" require parents to obtain a standardized form that must be obtained from the local health department, attend a vaccine education session with a school nurse, or compose a statement of their objection to immunization on religious or philosophical grounds; "difficult states" require parents to complete a standardized form or write a letter explaining their opposition to immunization as well as having the form or letter notarized by a notary public.

States with easier opt-out requirements, non-medical exemption rates were found to be more than twice as high as those in states with more complex requirements. This led researchers to believe that states with easy exemption processes do in fact have lower vaccination rates, thereby contributing to outbreaks of measles and other childhood infections in these areas.

"It appears that increasing numbers of parents across the country desire not to vaccinate their children out of fear, convenience, or misinformation," says lead author, Nina R. Blank, a research associate in the Division of Medical Ethics at NYU Langone. "We believe that states with very loose non-medical exemption requirements make it easier for parents to opt-out of immunizations and may pave the way for them to send their children to school unvaccinated, potentially putting other children at risk."

A more ethical approach, the researchers say, may exist in California, Vermont, and Washington where, by law, vaccine-hesitant parents are required to obtain immunization education before an exemption will be granted. Additionally, health departments in states such as Kansas, Rhode Island and the District of Columbia encourage yearly educational sessions with school nurses for parents reluctant to have their children immunized.

"Based on our findings, it seems that policy makers seeking to decrease opt-out rates should add vaccine education components to the exemption process by either mandating or encouraging yearly educational sessions in schools for parents who are reluctant to have their children vaccinated," said Blank.

Catherine Constable, MD, a medical resident in the Department of Medicine, NYU Langone Medical Center, contributed to the study.

The authors acknowledge the assistance provided by the Center for Vaccine Ethics and Policy, a joint program of the Division of Medical Ethics, NYU Langone Medical Center; the Wistar Institute Vaccine Center; and the Vaccine Education Center at the Children's Hospital of Philadelphia.