Missing some doses of the diptheria, tetanus toxoid and acellular pertussis (DTaP) vaccine series increases whooping cough risk in young children, results of a case-control study indicate.
Children aged 3 to 36 months who missed three doses of DTaP were almost 19 times more likely to develop pertussis than those who were appropriately vaccinated, and those who missed four doses were 28 times more likely, Jason Glanz, PhD, of Kaiser Permanent Colorado, and colleagues reported in JAMA Pediatrics.
Recent pertussis outbreaks across the United States highlight a trend towards undervaccination spurred by a variety of reasons, including parental choice, barriers to healthcare and medical contraindications. To examine the association between undervaccination and pertussis, Glanz and colleagues analyzed data from children belonging to eight managed care organizations in the Vaccine Safety Datalink from 2004 to 2010.
There were a total of 72 laboratory-confirmed pertussis cases. The researchers matched each case with four randomly selected controls (N=288). Undervaccination was defined as missing or delaying one or more of the first four doses in the DTaP vaccine series.
Among the 72 pertussis cases, 47.22% were undervaccinated for DTaP (N=34) compared with 22.22% of the 288 matched controls.
Among undervaccinated children who developed pertussis, nearly one third (29.41%) had an administrative code for parental vaccine refusal or delay, suggesting parents “had intentionally refused or delayed vaccine dose for personal, nonmedical reasons,” the researchers wrote.
The risk for pertussis increased in a dose-response relationship, with the likelihood of disease becoming greater as the number of missed doses increased:
- One missed dose: odds ratio 2.25, 95% CI: 0.97-5.24, P=0.06
- Two missed doses: OR 3.41, 95% CI: 0.89-13.05, P=0.07
- Three missed doses: OR 18.56, 95% CI: 4.92-69.96, P<0.001
- Four missed doses: OR 28.38, 95% CI: 3.19-252.63, P=0.002
About 36% of pertussis cases among children aged 3 to 38 months could have been prevented if children were vaccinated on time with DTaP, the researchers calculated.
Not surprisingly, we found that the risk of pertussis greatly increased as undervaccination with DTaP vaccine increased,” the researchers wrote. “We believe that our study supports on-time vaccination with DTaP vaccine, as recommended by the Advisory Committee on Immunization Practices.”