Undervaccination increases pertussis risk in children three months to three years of age. Children who miss three or four doses of the diphtheria, tetanus, and acellular pertussis (DTaP) vaccine are 18 and 28 times more likely, respectively, to be diagnosed with pertussis (whooping cough) than children who are fully vaccinated, reported Glanz and colleagues online September 9 in JAMA Pediatrics. Records of children born between 2004 and 2008 at eight managed care organizations identified 72 cases of pertussis; 47% of those children didn't receive the recommended four doses of the DTaP vaccine. In contrast, only 22% of children not diagnosed with pertussis were undervaccinated. The DTaP vaccine is recommended at two, four, and six months and again at 15 to 18 months and at four to six years of age. Undervaccination, whether a result of parents' refusal or barriers to health care, has contributed to the largest outbreak of pertussis in the United States in 50 years-which is occurring now.