Epidemiologic Notes and Reports Pertussis Surveillance -- United States, 1984 and 1985

During the period 1984-1985, a provisional total of 5,865 pertussis cases was reported to the MMWR (2,276 in 1984, 3,589 in 1985*), for an average crude incidence rate of 1.2 cases/100,000 total population (0.96/100,000 in 1984 and 1.5/100,000 in 1985*). This is a 33% increase over the average rate reported for 1982 and 1983 (0.9/100,000). Pertussis cases were reported from all 50 states and New York City, with the highest average annual attack rates reported in Oklahoma (6.9/100,000), New Hampshire (6.4/100,000), Alaska (5.8/100,000), Wisconsin (5.2/100,000), Indiana (4.4/100,000), and Maryland (4.4/100,000). Age-specific attack rates were highest among children 1 year of age and declined with increasing age (Figure 1). The incidence of reported pertussis in all age groups has increased since 1981. However, the greatest increases have been in persons greater than or equal to 15 years of age. From 1981 to 1985, a 13-fold increase in incidence from 0.02/100,000 to 0.25/100,000 was observed for persons greater than or equal to 20 years of age.

Supplemental case report forms** were received on 5,145 patients with onsets of illness during this 2-year period, representing 88% of the total number of cases reported by state health departments to the MMWR. In contrast, report forms were received on 77% of the pertussis patients reported to the MMWR in 1982 and 1983.

Individual case report forms were analyzed on 4,728 patients from all 50 states and New York CityS. The states submitting the largest number of case report forms were Indiana (449), Oklahoma (423), Texas (412), Wisconsin (411), and Washington (364). The age distribution of pertussis patients whose forms were received was similar to that of patients with known ages reported to the MMWR, suggesting that the forms received were representative of the total reported cases (Table 3).

Laboratory confirmation was reported for 3,317 (70%) patients; 20% of these were confirmed by both culture and direct fluorescent antibody (DFA) testing of nasopharyngeal secretions; 19%, by culture only; and 61%, by DFA only. Fifty percent (2,328) of patients had onsets of cough during the months of June through September. Patients with cases reported to the surveillance systems tended to have classic clinical symptoms. Whoop was described in 47% of patients (53% of those 0-4 years old and 44% of those 5-9 years old). Paroxysms were reported in 79% of patients; apnea was reported in 35%.

Rates of hospitalization and other complications were highest in children 6 months old and declined with increasing age (Table 4). Death was reported in 19 (0.4%) patients, 18 of whom were less than or equal to 6 months of age. The case-fatality ratio in this age group was 1%. Thirteen of the deaths occurred in patients with pneumonia, two of whom also had encephalopathy.

Based on age criteria derived from the Immunization Practices Advisory Committee (ACIP) recommendations concerning diphtheria, tetanus toxoids, and pertussis vaccine (DTP), children are appropriately immunized for their age if they have received one dose by 3 months of age, two doses by 5 months of age, three doses by 7 months of age, and four doses by 19 months of age (1). Of 1,504 patients 7 months through 6 years old with known vaccine status, 70% were not appropriately immunized, and 55% had not received at least three doses of vaccine--the minimum number considered necessary for adequate vaccine protection against pertussis. Thirty-one percent had not received any doses. Additionally, 405 (51%) of the 795 reported patients 3-6 months of age with known vaccine status were not appropriately immunized with DTP vaccine for their age.

Ninety-five percent of the 4,544 persons with known antibiotic status received antibiotic therapy. Seventy-five percent (3,236) of patients with known antibiotic type received erythromycin; 19% (813) received ampicillin/amoxicillin; and 6% (247) received other types of antibiotics. Of those treated with erythromycin, 68% started therapy within 14 days of the onset of cough; 85%, within 21 days; and 91%, within 28 days. Reported by: State and Territorial Epidemiologists. Div of Immunization, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: The supplemental pertussis surveillance system was introduced in 1979, with the cooperation of the Council of State and Territorial Epidemiologists, to provide additional information on pertussis epidemiology, health impact, and vaccine and antibiotic usage and efficacy. Since its introduction, state participation has progressively increased, as witnessed by the increasing percentage of patients who have been reported both to the MMWR and to the supplemental pertussis surveillance system. Individual case report forms were submitted on 20% of patients in 1979, compared with 88% in 1985.

While the crude annual incidence rate of pertussis reported to the MMWR increased three fold from 1981 to 1985 (0.5/100,000 to 1.5/100,000), it is still 3% of the average annual pertussis incidence rate reported during the prevaccine era, 1922-1948. The age-specific incidence of pertussis for all age groups increased during this period. Most notably, persons greater than or equal to 20 years of age showed a marked increase in pertussis incidence throughout the period.

The overall increase in reported cases of pertussis may represent an actual increase, a reporting artifact, or a combination of both. Reporting may have improved because of: 1) an increase in awareness of disease by both professional and lay persons, 2) an increase in awareness by physicians that pertussis can occur in adult patients, 3) an increase in diagnostic capability of laboratories, and 4) an increase in case investigations by health departments. The current contribution of each of these factors to the increase in incidence of reported cases is unknown.

The supplemental pertussis surveillance system and the MMWR receive reports on a disproportionate number of pertussis patients who are hospitalized or laboratory-confirmed or have classic clinical disease. While reporting of non-pertussis cases as pertussis may be a problem, the fact that substantial proportions of cases have laboratory confirmation and classic clinical manifestations means such a reporting error is not likely to be a major problem. However, like other passive surveillance, pertussis surveillance suffers from low sensitivity because of underdiagnosis and underreporting. This is further complicated because, among the states, there is no uniform case definition or laboratory criteria for diagnosing and reporting pertussis. As of 1984, only 40% of states had established a case definition for the diagnosis of pertussis; 50% did not count physician-diagnosed cases if laboratory studies were not done; and 60% did not report physician-diagnosed cases if laboratory studies were done but were negative (2). Nevertheless, the pertussis surveillance systems provide estimates of the minimum incidence of pertussis, the maximum rates of disease complications, and the health impact of pertussis. They also provide comparisons of trends in disease epidemiology and the use of prevention and control measures.

Most of the cases could potentially have been prevented by proper vaccination. Studies have shown repeatedly that pertussis vaccine has been highly effective in preventing disease (3). As in previous years, almost 70% of pertussis patients 3 months to 6 years of age were not appropriately immunized with DTP, and a third of all patients 7 months to 6 years of age had not received any vaccine.

Serious neurologic illnesses requiring hospitalization attributable to pertussis vaccine are rare. Final analysis of a comprehensive case-control study has estimated that the risk of such illnesses is 1 in 140,000 doses administered (4). An earlier analysis had estimated this risk at 1/110,000 doses (5). In contrast, final analysis of the case-control study found that the risk of serious neurologic illness following pertussis disease was 1/11,000 pertussis cases (4). Repeated evaluations have shown that the benefits of vaccine outweigh the risks (6,7). Therefore, both the ACIP and the American Academy of Pediatrics continue to recommend the routine use of DTP vaccine.

References

1. ACIP. Diphtheria, tetanus, and pertussis: guidelines for vaccine prophylaxis and other preventive measures. MMWR 1985;34:405-14, 419-26.

2. Sacks JJ. Utilization of case definitions and laboratory reporting in the surveillance of notifiable communicable diseases in the United States. Am J Public Health 1985;75:1420-2.

3. CDC. Pertussis--United States, 1982 and 1983. MMWR 1984;33:573-5.

4. Miller D, Wadsworth J, Diamond J, Ross E. Pertussis vaccine and whooping cough as risk factors in acute neurological illness and death in young children. Develop Biol Standard 1985;61:389-94.

5. Miller DL, Ross EM, Alderslade R, Bellman MH, Rawson NSB. Pertussis immunisation and serious acute neurological illness in children. Br Med J 1981;282:1595-9.

6. Hinman AR, Koplan JP. Pertussis and pertussis vaccine: reanalysis of benefits, risks, and costs. JAMA 1984;251:3109-13.

7. Hinman AR, Koplan JP. Pertussis and pertussis vaccine: further

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