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Epidemiologic Notes and Reports Pertussis Outbreak -- Oklahoma

From January 1, through December 10, 1983, 330 cases of pertussis were reported from Oklahoma. Detailed analysis of the first 237 patients, with onsets of illness through August 28 and reported through September 16, is presented below. All met the following case definition: illness (1) confirmed by culture and/or direct fluorescent antibody test (FA) for Bordetella pertussis, (2) diagnosed as pertussis by a physician, or (3) characterized by a cough of 2 weeks or more and epidemiologically linked to a laboratory-confirmed or physician-diagnosed case.

The diagnosis of pertussis was confirmed by culture and/or FA in 123 (52%) of the 237 cases (16% by culture, 26% by FA, 10% by both). The first reported patient in 1983 had onset of cough on January 9. From January through April, the number of reported cases occurring weekly was fairly constant (Figure 1); in May, however, a marked increase occurred. Cases peaked during June and July. From January 1 to September 15, cases were reported from 27 (35%) of 77 of the state's counties.

Eighty-nine (38%) cases occurred among children under 1 year old (Table 1). Of these, 62 (70%) occurred among children less than 6 months of age; 20% were 15 years of age or older. One hundred forty-three (60%) cases occurred among children 3 months through 6 years of age, the target group for pertussis vaccination. Of the 136 patients in this age group with known immunization status, 49 (36%) were up-to-date for age for diphtheria and tetanus toxoids and pertussis vaccine (DTP) (Table 2) according to criteria derived from the Immunization Practices Advisory Committee (ACIP) recommendations.*

Sixty-eight (29%) ill individuals were hospitalized. The hospitalization rate for patients under 1 year of age was 56% (50/89). Of these, 39 children were under 6 months of age. Persons with pertussis aged 3-11 months whose vaccinations were not up-to-date for age were 4.6 times more likely to be hospitalized than those who were up-to-date. Five (18%) of the 28 hospitalized patients 3-11 months of age were up-to-date for DTP immunization, compared with 20 (91%) of the 22 nonhospitalized patients, (p 0.001). No deaths were reported. Two unimmunized children, aged 1 month and 3 months, experienced seizures; both were hospitalized, and neither had apparent neurologic sequelae at the time of discharge.

Detailed clinical information was available for 19 children admitted to a hospital in Oklahoma City. All were under 3 years of age; 14 (74%) were less than 6 months of age and, therefore, too young to have received three doses of DTP according to the routine schedule recommended by the ACIP. Three children (16%) were admitted to the intensive care unit; seven (37%) had radiologic evidence of pneumonia, and two (11%) (previously cited) had seizures. The hospital stay for the 19 children ranged from 1 to 51 days (mean 10 days).

The largest number of cases (57) occurred in Oklahoma County and were scattered diffusely throughout the county without evidence of geographic clustering. Chains of transmission of illness were identified in 26 instances involving 136 cases. When cases could be linked, babysitting or extended-family settings were the most common mode of contact between households. In 47 instances of interhousehold transmission, 38 (81%) occurred in such settings. Localized transmission in neighborhoods did not appear to play an important role. A survey on August 31 of 150 homes around the residences of three pertussis patients uncovered no evidence of house-to-house transmission.

During the Oklahoma County neighborhood survey, the immunization status of children 3 months through 6 years of age was assessed. None was unvaccinated; of 57 surveyed, 37 (65%) were up-to-date for age for DTP vaccination. Concern about the adverse effects of pertussis vaccine was not cited as a reason for nonvaccination.

The population in the neighborhoods of patients was primarily served by public clinics. Immunization records of a 10% sample of the approximately 5,300 children under 7 years of age attending the main Oklahoma County clinic were reviewed on September 3 for DTP immunization to determine if the low immunization levels reflected the immunization status of children county-wide in public clinics. One hundred forty-four (27%) of 526 attendees were up-to-date. Of those 3 months through 18 months of age, 109 (49%) of 222 were up-to-date.

Measures to control transmission in accordance with ACIP recommendations were utilized throughout the outbreak. These consisted of: (1) administration of erythromycin to individuals with confirmed or suspected pertussis (to shorten the period of infectivity) if such therapy could be initiated within 3 weeks of cough onset, (2) DTP vaccination of close contacts under 7 years of age who were not up-to-date for DTP vaccinations, and (3) erythromycin chemoprophylaxis of close contacts under 1 year of age and incompletely immunized close contacts under 7 years of age.

Because the outbreak had spread throughout Oklahoma County, an effort directed toward large segments of the population appeared necessary. The principal target population consisted of persons under 7 years of age who might be rendered immune with a single dose of DTP. Assuming at least three doses of pertussis vaccine are needed for protection, 40 (17%) of the reported cases could potentially have been prevented by vaccination with a single additional dose--11 (5% of all patients) had received three doses, and four (2%) had received four doses but were not up-to-date; 25 (11%) had received two prior doses. Assuming that two doses might offer some level of protection, an additional 34 cases (14%) with only one prior dose might potentially have been prevented. Thus, 32% of cases might have been directly prevented by receipt of one additional dose in an outbreak control program. The potential indirect effect of eliminating these cases on the overall transmission of disease is unknown.

Since the survey in the patients' neighborhood indicated that 70% of children received their immunizations from public clinics, the outbreak control effort was directed toward clients of county clinics. A complete audit of immunization records of children under 7 years old attending the main Oklahoma County clinic was performed and extended to two of the four other county clinics. Initially, letters were mailed to parents of 931 children not up-to-date or due for a DTP dose residing in one quadrant of the county. Special day and evening immunization clinics were run for a 2-week period beginning September 22. During that period, 137 (15%) of the targeted children returned for immunization; parents of 25 (3%) called to indicate that DTP had been administered elsewhere; and 183 (20%) of the letters were returned as undeliverable. Notices for children not up-to-date or due for a DTP dose in the remainder of the county were subsequently mailed. Pertussis transmission continued despite the clinics. Through December 10, 40 additional cases were reported in the county. Reported by G Dellaportis, MD, S Winn, MPH, G Redgrave, P Caldwell, C Matthews, Oklahoma County Health Dept, all other local and county health departments in Oklahoma; PJ Rettig, MD, R Corrie, M Ehn, G Ryan, Children's Memorial Hospital, Oklahoma City, P McKee, P Callihan, J Dudley, G Harper, P Wall, C Enright, J Williams, Immunization Div, GR Istre, MD, State Epidemiologist, Oklahoma State Dept of Health; Surveillance, Investigations, and Research Br, Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: From 1975 to 1982, the annual number of reported pertussis cases in the United States varied from a low of 1,010 in 1976 to a high of 2,177 in 1977. The 28% increase in reported cases through December 10, 1983 (2,134), compared to a similar period in 1982, is not inconsistent with this pattern. However, it is not possible to determine whether the nationwide increase of reported pertussis cases in 1983 over 1982 reflects a true increase or improved recognition and/or reporting following recent lay and medical media coverage of the risks and benefits of pertussis vaccine (1).

The Oklahoma outbreak represents the largest number of reported cases in that state since 1956. The hospitalization rate and the complication rate for hospitalized children verify that pertussis can be severe, with substantial health impact, particularly in the very young (2). Furthermore, the lower hospitalization rates in patients who were up-to-date for DTP immunization, compared with those who were not up-to-date, corroborates findings of other reports that vaccinees who develop disease tend to have less severe illnesses (2-4).

Low immunization levels in children appear to have been a major factor associated with this outbreak. Three or more doses of DTP prevent disease in approximately 80% of recipients (2). High DTP coverage of children through 6 years of age (including a booster dose at 4-6 years of age) indirectly protects the highest risk group--infants less than 6 months of age who are too young to have routinely received at least three doses of DTP. Although the ACIP recommends that the first three DTP doses should be 4-8 weeks apart, the routine interval is approximately 8 weeks. In an outbreak

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