A child killer is loose again: A new study shows that whooping cough is on the march, especially in states that make it easy for parents to reject vaccination.

California and 18 other states allow parents to exempt their children from vaccination requirements based on personal beliefs; other states allow a non-medical exemption only for religious beliefs.

Easy-exemption states had about 50 percent more cases than in less flexible states, concluded the Johns Hopkins study, published in the Journal of the American Medical Association.

Whooping cough, also known as pertussis, is a highly contagious respiratory infection spread by coughing. It can lead to pneumonia, seizures, brain damage and death in young children and babies.

States should toughen rules requiring children to be vaccinated before starting school and launch public-health campaigns to convince parents the pertussis vaccine is safe and highly effective.

Once a common childhood disease, whooping cough was nearly wiped out when the pertussis vaccine went into widespread use in 1948.

Immunity wears off after about 10 years. Teens and adults don't get dangerously ill from whooping cough, but they can infect young children who haven't been immunized or haven't completed the series of shots (usually called DPT shots because the pertussis vaccine is combined with diphtheria and tetanus protection). Public health experts now recommend booster shots every 10 years, especially for anyone in close contact with babies.

But some parents believe unwisely that vaccines pose a greater risk than the diseases they prevent. In countries where anti-vaccine campaigns limited coverage, whooping cough has become epidemic again. Worldwide, the disease kills an estimated 300,000 children a year.

Whooping cough is posing a growing threat here: In 1976, there were only 1,010 cases in the United States. That soared to 25,827 cases in 2004, according to the Centers for Disease Control and Prevention.

In states like Mississippi, which makes it hard to get an exemption, there are fewer than one case per 100,000 people; Vermont, an easy-exemption state, averages 13 per 100,000 people. California ranks in the middle in cases and deaths: Seven children died of whooping cough in the state in 2005. Too many.

Persuading parents to accept vaccination and making it harder for them to leave their babies vulnerable would prevent needless deaths.