In an analysis of US vaccine-exemption laws, investigators found higher rates of whooping cough in states where parents can refuse to vaccinate their child due to "personal beliefs."

The disease rate in these states was about 50 percent higher than it was in states that only allowed exemptions for medical reasons and religious beliefs, the researchers report in the Journal of the American Medical Association.

The same was true of states with "easy" exemption procedures, according to the study authors, led by Saad B. Omer of Johns Hopkins University in Baltimore.

This includes states such as California, where parents can take a personal-belief exemption by simply signing a school immunization form. Other states, such as Maryland, officially allow only religious exemptions; but again, parents have only to sign a form, making it likely that many take the exemption for personal reasons.

The elevated rates of whooping cough in these states point to the "very real consequences" of relaxing vaccination requirements, Omer said in a statement.

Also known as pertussis, whooping cough is a highly contagious bacterial infection of the respiratory system that causes fits of severe coughing and breathing difficulties -- often with a distinctive "whoop" sound on inhalation. People of any age can become infected, but it's most dangerous, and potentially fatal, in babies and young children.

Childhood vaccination with the combined diphtheria, tetanus and pertussis vaccine can prevent whooping cough, but the rate of infection in the US has been climbing in recent years.

This trend is one reason the current study was undertaken, said Dr. Daniel A. Salmon, an associate professor of epidemiology at the University of Florida College of Medicine and the study's senior author.

All US states require children entering school to have proof they've received standard vaccinations, though all also grant exemptions for medical reasons. In addition, nearly all states also allow exemptions for religious beliefs, while 19 grant waivers for personal beliefs.

In these latter states, more and more parents have been opting out of vaccination in recent years, Salmon and his colleagues found. On average, the rate of non-medical exemptions grew by 6 percent per year between 1991 and 2004.

Concerns about vaccine safety seem to be the main reason parents claim such exemptions, Salmon told Reuters Health. In an earlier study, he and his colleagues found that 69 percent of parents who sought exemptions did so because they feared vaccination did more harm than the diseases it prevents.

In part, such concerns stem from the proposed link between the measles/mumps/rubella (MMR) vaccine and autism -- a link that a number of international studies have since refuted.

Salmon and his colleagues argue that states should have "administrative controls" that make non-medical exemptions more difficult to obtain. This, Salmon said, could look something like the process of becoming a conscientious objector to the draft.

Parents would apply for an exemption and have to show a "strongly held belief" against vaccines, he explained. Then the government would either have to demonstrate an "overwhelming need" for universal vaccination or grant the exemption.