From January 1, to October 1, 1984, 162 cases of pertussis were reported from Seattle-King County, Washington (Figure 1). Before the outbreak, Seattle-King County's annual average number of reported cases (1979-1983) was 12; the state averaged 40 cases. The peak in reported cases in August and September coincided with the institution of aggressive active surveillance. The Seattle-King County cases were classified as: (1) confirmed--positive nasopharyngeal culture with any respiratory symptom (30 cases); (2) probable--positive fluorescent antibody (FA) test with any respiratory symptom but without positive culture (79 cases); or (3) possible--symptomatic without other known cause plus exposure to a definite or probable case but lacking laboratory confirmation (53 cases). Of the confirmed cases, 23 (77%) were also FA positive. Of an additional 63 asymptomatic individuals with positive FA tests, one also yielded a positive culture.

Twelve (7%) of the 162 patients were hospitalized; 10 of these were under 6 months of age. The hospitalization-to-case ratio among patients under 6 months of age was 45% (10/22). One 6-week-old patient had x-ray-documented pneumonia. No pertussis deaths were reported. The female-to-male ratio was 1.2:1.

Thirty-three cases (20%) were among children under 1 year of age (Table 1); of these, 22 children (67%) were under 6 months of age. Seventy-one (44%) cases occurred among patients 7 years of age or older; 78 (48%) cases occurred among children 3 months to 7 years of age, a group that should have received at least one dose of pertussis vaccine. Of the 69 patients 3 months through 83 months (6 years) of age with known immunization status, 34 (49%) were appropriately immunized for their ages with diphtheria and tetanus toxoids and pertussis vaccine (DTP)* (Table 2). There were no significant differences between age groups in the proportion of patients appropriately immunized. Among patients 7 months through 83 months of age and, therefore, old enough to have received at least three doses of DTP, 42 (70%) of the 60 with known immunization status had received three or more DTP doses.

The following control measures were undertaken: (1) immunization of children was urged through the news media; (2) a physician's advisory was distributed advising immunization of previously inadequately immunized children under 7 years of age, encouraging consideration of pertussis in the differential diagnosis of illness with cough, and asking that cases be reported promptly; (3) in the outbreak setting, the primary DTP immunization schedule was accelerated, with the first three doses recommended at 1H, 2H, and 3H months of age, compared to the usual practice of administration at 2, 4, and 6 months of age; (4) erythromycin treatment of suspected cases and prophylaxis of household contacts for 14 days was recommended regardless of contact age or immunization status (for those unable to tolerate erythromycin, trimethoprim/sulfamethoxazole was recommended); and (5) exclusion of patients from school or work until completion of 7 days or more of antibiotics was suggested.

Three studies were conducted during the outbreak: Church group investigation. On August 29, questionnaires were completed for all 161 members of the 44 households belonging to a church from which five probable pertussis cases had been reported. Two additional asymptomatic FA-positive members, one of whom was also culture-positive, had also been reported. Excluding the seven index members, specimens for FA smears and cultures were obtained from 93 (60%) members; 41 (44%) specimens were FA positive. Five percent of both FA-positive and FA-negative members had at least one respiratory symptom (Table 3). Aside from the one culture-positive index member, no other cultures were positive.

One month later, follow-up questionnaires of the 88 asymptomatic members showed that 18% and 24% of the FA-positive and FA-negative members, respectively, had developed respiratory symptoms in the interim. Forty-three percent of 44 FA-positive members for whom prophylactic erythromycin were prescribed indicated compliance.** Information was available on both symptoms and antibiotic compliance for 30 initially asymptomatic FA-positive members for whom an antibiotic was prescribed. There was no significant difference in the development of symptoms between those who complied (4/17) and those who did not comply (0/13) with antibiotic therapy (p = 0.09).

One or more members of 22 of the 44 households were FA positive. Specimens were available on all 69 household members of 19 of the 44 households. In these 19 households, FA-positive members were more likely to be grouped within a household than was expected by chance alone (p 0.01). FA positivity was not related to age, sex, household size, presence of small children in the household, church study group, previous disease history, exposure history, or vaccination status.

Potential workplace occurrence of adult-to-adult transmission. Evidence for adult-to-adult transmission was sought in each of three consecutively reported indoor workplace situations with a laboratory-positive pertussis case (one confirmed and two probable) and in a fourth workplace situation with a possible case. These workplace situations met the following criteria: (1) a symptomatic pertussis patient 18 years old or older; (2) one or more adults sharing the workplace; and (3) a workplace area the same size or smaller than an average school classroom area. The situations were a vanpool and three offices with eight, three, five, and seven persons, respectively, exposed to a reported case. Questionnaires were completed and specimens collected on 91% of the 23 co-workers. Transmission occurred only in the vanpool, where four (50%) of eight exposed adults developed apparent pertussis (one confirmed, one probable, and two possible cases). Active surveillance of an additional 32 persons in adjoining offices in two of the workplace situations did not detect any additional cases.

The possibility of secondary transmission by co-workers to members of their households was also studied. Questionnaires and specimens were obtained from 95% of the 37 household members of co-workers; none developed clinical pertussis. However, no household had children with immunization histories of less than three doses of pertussis vaccine. Aside from the one culture-positive contact case in the vanpool, all culture and FA specimens from contacts were negative.

Effectiveness of immunization recall. A search of the records of 18,059 children under 2 years of age who had attended county public health clinics identified 2,301 (13%) children who were eligible for a DTP dose by the accelerated immunization schedule. Letters describing the epidemic and urging immediate immunization were sent September 26-27 to parents of 2,211 (96%) of these children. During the next 2 weeks, 427 (19%) children received a DTP dose at the clinics. Supplementary telephone calls were then made over 2 days to 263 (15%) of the remaining 1,784 nonrespondents selected by systematic interval sampling. During the following 2 weeks, 36 (14%) of those who received phone calls responded, compared to 163 (11%) of 1,521 of those who did not receive phone calls (p

0.5). Including those who received phone calls, only 626 (28%) of the identified children returned to the clinics for immunization during the 4 weeks following the mailing.

Using systematic interval sampling, 59 responders and 57 nonresponders were selected, and telephone interviews were conducted with their parents. While 57 (97%) of 59 parents of responders knew of the epidemic, only 10 (18%) of 55 credited the letter, and one (2%) of 55, the media, as the stimulus for bringing the child in for immunization. Among parents of nonresponders, 53 (95%) of 56 knew of the epidemic. Households of nonresponders were more likely to have a primary wage-earner other than the father (p = 0.03), with a high school education or less (p = 0.05), a mother under 25 years old (p = 0.01), a longer travel time required to reach the clinic (p 0.01), and a lower income (p 0.05). Nonresponse was not associated with race, household size, years at current address, or employment status of the person responsible for taking the child for medical care. Reported by M Haupt, D Marinig, S Sumida, PhD, Virginia-Mason Medical Center, S Helgerson MD, K Johnson, MPH, J Boase, MS, L Kamahele, Seattle-King County Dept of Public Health, R Finger, MD, J Kobayashi, MD, State Epidemiologist, Washington Dept of Social and Health Svcs; Div of Field Svcs, Epidemiology Program Office, Surveillance, Investigations, and Research Br, Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: The 2,463 reported cases of pertussis in the United States in 1983 and the 2,187 cases provisionally reported in 1984 are the largest annual numbers of reported cases since 1974 (1975-1984 annual average: 1,813 cases). Supplementary information on reported cases from 1979 to 1983 indicates that children under 6 months of age are at greatest risk of disease morbidity, severity, and mortality (1,2); 14% of patients in the Seattle-King County outbreak were under 6 months of age, of whom 45% were hospitalized. Because at least

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