Children who are not vaccinated according to the schedule recommended by U.S. health officials are at an increased risk of catching whooping cough, according to a new study.

Researchers found kids who fell significantly behind on their diphtheria, tetanus and acellular pertussis (DTaP) shots were between 19 and 28 times more likely to be diagnosed with whooping cough, also known as pertussis, than children who were vaccinated on time.

“What we found - not surprisingly - was that kids who were not vaccinated on time were at a greater risk of pertussis, compared to those who were vaccinated on time,” Jason Glanz, the study’s lead author from the Kaiser Permanente Colorado Institute for Health Research in Denver, said.

Studies have shown pertussis cases have been on the rise across the U.S.

Researchers suspect that’s due to the use of a new type of pertussis vaccine - which has fewer side effects, but is less effective over the long run - and to more children missing or delaying vaccination (see Reuters Health story of May 20, 2013 here: reut.rs/18aYu17.)

Although parents sometimes believe delaying their children’s vaccines and following alternative schedules is safer, Glanz said there is no evidence that they are “any safer in terms of adverse events than what’s typical.”

For the new study, he and his colleagues compared the vaccination records of 72 children who were diagnosed with pertussis in one of eight healthcare systems between 2004 and 2010 and 288 similar kids who didn’t get the disease. All of the children were between the ages of three and 36 months.

The Centers for Disease Control and Prevention recommends children receive doses of the DTaP vaccine at two, four and six months of age, another dose between 15 and 18 months and a booster shot when they’re four to six years old.

The researchers found that about 47 percent of children diagnosed with pertussis were not vaccinated according to that recommended schedule, versus about 22 percent of kids in the comparison group.

“Just over a third of the cases could have been prevented had they been vaccinated on time,” Glanz said.

His team also found that the longer parents delayed getting their children vaccinated, the higher the kids’ risks of catching pertussis climbed.

Those who were three doses behind, for example, were 19 times more likely to get whooping cough than kids who were caught up on their shots, and those who were four doses behind had 28 times the risk.

“I think the nice thing about this study is that it quantifies the risk,” Dr. Mary Healy, director of vaccinology and maternal immunization for the Center for Vaccine Awareness and Research at Texas Children’s Hospital in Houston, said.

“Pertussis is a particularly serious disease. The difficulty with pertussis is that it’s very often underappreciated how serious it can be,” Healy, who was not involved with the study, said.

In an ongoing pertussis outbreak in Texas, about 2,000 cases have been reported in the state this year. So far, two infants, who were too young to receive the DTaP vaccine, have died (see Reuters story of September 5, 2013 here: reut.rs/1dWKX3J.)

Healy told Reuters Health it’s also important for adults including pregnant women in their third trimester to get booster shots of the vaccine to protect young children and those with weakened immune systems.

Glanz said common side effects of the vaccine include fever, discomfort and a sore arm. Serious side effects are rare, he said, adding that parents should understand that delaying their kids’ vaccines puts the children at a greater risk of catching pertussis.

“The purpose isn’t to demonize parents or marginalize them. We’re doing this not because we’re pro-vaccine but because we’re pro-child,” he said.

SOURCE: JAMA Pediatrics, online September 9, 2013.

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