One in 20 children whose parents do not get them vaccinated against whooping cough catch the highly contagious virus, researchers said on Tuesday.

Only one in 500 children who got vaccinated developed whooping cough in the study by Kaiser Permanente Colorado's Institute for Health Research.

The researchers said their government-funded study was the first to use medical records to confirm which children received immunizations and which ones did not — and the subsequent rates of whooping cough, or pertussis.

The illness, which features uncontrollable deep coughing, is rarely fatal but infants are most vulnerable and accounted for most of the 140 U.S. deaths from pertussis between 2000 and 2005, said Dr. Jason Glanz, an epidemiologist who worked on the study published in the journal Pediatrics.

Increasingly, fears about possible health threats such as autism from childhood vaccinations have led a small proportion of parents to refuse the shots for their children. That has raised the prospect that vaccine-preventable illnesses may reappear in clusters.

Immunization has been credited with the eradication or control of smallpox, polio, diphtheria, measles, mumps and rubella.

But whooping cough is still entrenched, with 10,000 U.S. cases in 2007, so it was chosen to study the impact of vaccine refusals, Glanz said in a telephone interview.

"The U.S. immunization program has been so successful at eliminating so many diseases that we no longer see them. So it's essentially become a victim of its own success.

"Now parents are no longer worried about the diseases themselves, as far as their children getting sick. They're more worried about vaccine safety," he said.

Parents wrongly rely on "herd immunity" that assumes others will vaccinate their children so they do not have to, but the risk of an outbreak increases as more refuse to immunize, Glanz said. The "tipping point" to an outbreak may be if immunization rates drop below 90 percent, he said.

Most U.S. states allow schools to give medical exemptions for not getting a child vaccinated, and some states give religious exemptions or simply allow parents to refuse vaccinations out of a belief children face potential harm.

About 1 percent of U.S. parents get exemptions for their children, but the figure is higher in some areas, Glanz said.

Even if children survive a bout with whooping cough unscathed, they can spread illness-causing bacteria to more vulnerable victims, he said, including babies not yet vaccinated.

Vaccinations for whooping cough are given in five doses between 2 months and 18 months of age.

Glanz said commonly employed vaccines are tightly regulated and monitored and are safe — but he said that does not prevent misinformation from circulating on the Internet and by word of mouth about supposed dangers.

"I'm hoping that our study is an additional piece of information that doctors can use when conveying the risks and benefits," he said.

"There's a spectrum of parents that are concerned. There are some that are just staunchly anti-vaccine and there are a lot of concerned parents that are on the fence."