CHICAGO — State laws that make it easy for children to skip school-required vaccinations may be contributing to whooping cough outbreaks around the country, a study suggests.

All states allow children to be exempted from school immunization requirements for medical reasons and 48 states allow exemptions for personal or religious beliefs.

To get non-medical exemptions, some states require documentation, notarized paperwork and even visits to a local health department. In other states, parents merely have to sign an exemption letter.

Compared with stricter states, those with easy exemption policies had about 50 percent more whooping cough cases, according to the study. Also, about 50 percent more people got whooping cough in states that allowed personal-belief exemptions, compared with those allowing only religious exemptions.

States increasingly are being pressured to relax their exemption requirements, often by parents with unfounded fears about the risks of childhood vaccines, said University of Florida researcher Daniel Salmon, a co-author of the study. But loosening these policies would be a public health threat, he said.

The study appeared in Wednesday's Journal of the American Medical Association.

Researchers from Johns Hopkins University's Bloomberg School of Public Health contributed to the study, including two who reported financial ties to vaccine makers. Salmon said he has no financial connection to vaccine makers.

Whooping cough, also called pertussis, is a bacterial infection that causes severe coughing spells. It is highly contagious and can be deadly in infants. Reported cases have increased more recently, from 1,020 nationwide in 1976 to 25,827 in 2004. Public health officials believe the numbers are up because the vaccine's protection wears off. Booster shots are recommended.

Dr. Samuel Katz, a Duke University vaccine specialist who has consulted for vaccine makers, said he is not convinced loose state exemption policies are linked with whooping cough prevalence. He said not all states with liberal policies have high disease rates. But Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University who has also worked with vaccine manufacturers, said the connection is plausible.