DENVER (The New York Times News Service) -- Children whose parents refuse to have them immunized are 23 times more likely to get whooping cough, according to a study that is perhaps the most definitive yet linking vaccine refusal to disease.

The Colorado study, which spanned 12 years and included more than 700 children, is expected to become a weapon in the medical community's fight against a growing number of parents refusing vaccines for their children.

Immunization programs have been so successful that diseases such as polio and measles virtually have been eliminated, shifting concern instead to the safety of vaccines, said Jason Glanz, a study author and an epidemiologist with Kaiser Permanente's Institute for Health Research.

Parents refusing vaccines cite safety concerns as well as ineffectiveness.

"The perception the vaccine doesn't work isn't true," Glanz said. "We showed it clearly does work."

Glanz studied electronic medical records from 156 children who had whooping cough and 595 children who did not from 1996 to 2007.

One in 20 children whose parents refused the vaccine got whooping cough, compared with one in 500 vaccinated children.

A small percentage of children who are vaccinated against whooping cough still get the disease because they don't develop an immunity response or because their immunity wears off by adolescence. The whooping cough vaccine requires five shots, beginning at the age of 2 months and ending between 4 and 6 years.

The study, funded by the National Institute of Allergy and Infectious Diseases, appears in the June issue of the journal Pediatrics.

The highly contagious bacterial infection causes violent, uncontrollable coughing -- so intense that a long coughing fit often ends with a sharp intake of air, the "whoop."

Whooping cough, also called pertussis, is deadly in about 1 percent of infants younger than 2 months -- those who are too young to be vaccinated.

Many parents would prefer their children get whooping cough rather than suffer the consequences of a bad reaction to the shot, said Barbara Loe Fisher, co-founder of the National Vaccine Information Center, a vaccine "watchdog" group in Virginia. She faulted the study for not evaluating bad reactions to the vaccine or determining the reasons for parents' refusal.

Side effects that she links to the vaccine include high-pitched screaming and seizures. It's difficult to get medical exemptions for the vaccine, so many parents seek personal-belief exemptions, she said.

"When it happens to your child, the risks are 100 percent," Fisher said. "We need much better scientific studies to see who is at higher medical risk."

Colorado is among the states with the easiest exemption rules.

Parents only have to sign a form at the school nurse's office.

Before a whooping cough vaccine was developed in the 1940s, about 200,000 people each year in the United States came down with whooping cough and 10 percent of infants who got the disease died.

The deadliest year was 1923, when 9,000 people died.

Today, there are typically about 10,000 cases each year in this country and about 800 in Colorado.

The new version of the vaccine, developed in the 1990s, is "cleaner" and safer, said Dr. Sean O'Leary, a pediatric infectious disease fellow at Children's Hospital in Aurora. The original vaccine -- which some parents associate with neurological side effects -- was basically whooping cough bacteria, ground up and injected, he said.

The modern vaccine uses only a few proteins from the bacteria, he said.

Immunization rates continue to increase across the country, but so do the number of parents -- including actor couple Jenny McCarthy and Jim Carrey -- who believe vaccines are unsafe, linking them to autism and neurological diseases.

Doctors fear, though, that as more parents refuse vaccines, "herd immunity" will diminish, meaning diseases knocked out by vaccines could make a comeback. That's particularly worrisome for babies too young to receive vaccines and children who aren't vaccinated for health reasons.

"Vaccination is the victim of its own success," said Lydia McCoy, executive director of the Colorado Children's Immunization Coalition.

Diseases such as polio and measles are no longer "a visible threat" because vaccines are protecting people, she said.

In Colorado, 78 percent of children up to age 3 are fully vaccinated, according to 2007 statistics from the federal Centers for Disease Control and Prevention.

Copyright 2009 The New York Times News Service. All rights reserved.