Missed Vaccinations Increase Whooping Cough Risk in Young Kids

A child's risk of developing pertussis increased along with the number of doses of the diphtheria, tetanus, and acellular pertussis (DTaP) vaccine he or she had missed, a study finds.

Although the United States is currently experiencing the largest outbreak of pertussis in 50 years, the number of US children who do not receive all 5 recommended doses of the diphtheria, tetanus, and acellular pertussis (DTaP) vaccine is increasing. The results of a recent study suggest that this trend in undervaccination is contributing to rising rates of pertussis, also known as whooping cough.

According to the recommendations of the Advisory Committee on Immunization Practices, children should receive a total of 5 doses of the DTap vaccination, with doses administered at the following ages: 2 months, 4 months, 6 months, 15-18 months, and 48-83 months. Children who miss a dose or receive a dose late are considered to be undervaccinated. The study, published online on September 9, 2013, in JAMA Pediatrics, investigated the relationship between undervaccination and pertussis in children aged 3 to 36 months.

A total of 72 children born between 2004 and 2008 with a laboratory confirmed case of pertussis were enrolled from 8 managed care organizations of the Vaccine Safety Datalink and were matched with 288 healthy controls. Using health records, children were categorized as appropriately vaccinated for their age or undervaccinated. To evaluate a dose-response relationship, children were further categorized based on how many doses of the DTap vaccine they had missed, if any.

The results indicated a strong relationship between undervaccination and increased risk of pertussis. Just over 47% of children with pertussis were undervaccinated, compared with just 22.22% of children without pertussis. The researchers also found a relationship between the risk of pertussis and the number of doses children had missed. Children who had missed 3 doses of the vaccine were 18.56 times more likely to be diagnosed with pertussis than were fully vaccinated children. Children who had missed 4 doses were 28.38 times more likely to contract pertussis.

Children who missed 1 or 2 doses of the vaccine did not have statistically significant increases in risk compared with fully vaccinated children. However, the researchers note that this lack of significance may be due to limitations of the study design and was most likely affected by the age range of children included in the study. For example, if a child aged 21 months was undervaccinated by 1 dose, they would have previously received 3 doses, while an infant aged 3 months undervaccinated by 1 dose would have received no doses.

Although the recent outbreak of whooping cough in the United States is most likely due to a variety of factors, the researchers write, the findings indicate that undervaccination is contributing to the problem.

“We believe that our study supports on-time vaccination with DTap vaccine, as recommended by the Advisory Committee on Immunization Practices,” they conclude.