Risk factors for community- and household-acquired pertussis during a large-scale outbreak in central Wisconsin.

To identify risk factors associated with community- and household-acquired pertussis, we studied 61 households (HHs) with members with culture-positive illnesses and compared their characteristics with 58 neighborhood control-HHs and 62 randomly selected control-HHs. Case-HHs were more likely than either control group to have members 12-18 y of age (P less than .01); these individuals accounted for 34% of all primary cases. A history of exposure outside the home was the most important predictor of community-acquired infection (P less than .001), with adolescents being at higher risk than other age-groups (odds ratio, 3.2; P less than .001). After known exposure to a culture-positive case in the same HH, the risk of illness was unrelated to age; lengthy delays in initiating erythromycin therapy and prophylaxis were the only factors associated with secondary spread (P less than .01). The risk of pertussis may be related more to the likelihood of exposure than to age-related increases in susceptibility, and the risk can be reduced with appropriate use of erythromycin.