Outbreaks of pertussis in the United States: the Wisconsin experience.

To identify risk factors for pertussis in older age groups and to examine the effectiveness of erythromycin therapy and prophylaxis in reducing secondary spread, epidemiologic investigations of two outbreaks involving teenagers and adults were conducted. The first outbreak occurred in 1984 among residents of a facility for developmentally disabled persons (median age 17 years). Rates of culture-and/or serologically confirmed infection ranged from 6% to 91% in exposed wards (42% overall), with transmission continuing over a five-month period. Although residents age 10-19 years experienced the highest rates of infection, they were also more likely to be exposed compared with residents in other age groups. Infection rates were significantly lower on wards where erythromycin treatment and prophylaxis were initiated less than 2 weeks after onset of illness in the index case (overall attack rate = 16% vs. 75% in wards where more than 4 weeks had elapsed; p less than .0001). Early treatment with erythromycin was also effective in reducing pertussis severity. The second outbreak occurred over a six-month period among residents of a 3-county area in central Wisconsin in 1985, with adults accounting for 38% of 161 culture-positive cases. Exposure outside the home was the most important predictor of community-acquired infection (p less than .001), with adolescents being at higher risk than persons in other age groups (odds ratio 3.2; p less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)