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

All states allow children to be exempted from school immunization requirements for medical reasons – because they might have a bad reaction, for example, or have weak immune systems – and 48 states allow exemptions for personal or religious beliefs.

To get nonmedical 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, the study found.

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 appears in today’s Journal of the American Medical Assn. It was partly funded by the federal Centers for Disease Control and Prevention.

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.

The first whooping cough vaccine was licensed for U.S. use in 1948 and led to dramatic declines in disease. But 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 for teens and adults.

Salmon said liberal exemption policies may have contributed to the increase.

The highest average annual number of whooping cough cases from 1986 to 2004 was about 13 per 100,000 people in Vermont, a state with relatively loose exemption policies, the study found. The rate was well under one per 100,000 in Mississippi and several other states with stricter policies.

Dr. Samuel Katz, a Duke University vaccine specialist who has consulted for vaccine makers, said he is not convinced loose state exemption policies were linked with whooping cough prevalence. He said not all states with liberal policies had high disease rates.

But Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University who has worked with vaccine manufacturers, said the connection is plausible.

Schaffner said nonmedical exemptions should be allowed, but only if parents get educational information about vaccines. They also should be required to renew their exemption status each year, as some states now mandate, Schaffner said.