Pertussis Surveillance, 1979-1981

In 1979, in cooperation with state and territorial epidemiologists, CDC introduced a supplementary pertussis surveillance system to gather more detailed information on the epidemiology of pertussis in the United States. Before 1979, national pertussis surveillance by CDC was limited to data on the age and sex of each patient and the state reporting each case. These data were and continue to be reported in the Morbidity and Mortality Weekly Report (MMWR).

In the 3-year period, 1979-1981, the supplementary surveillance system received reports of 1,277 cases from 42 states. The largest numbers of cases were reported by Indiana (185), Washington (125), and New York (105).

Analysis of the data from this supplementary system indicates that 62% of the cases involved children 1 year of age, and that 79% of these cases were among children 6 months of age (Figure 1). The high percentage of cases found among 1-year olds by this supplementary system is similar to the 57% of pertussis cases in that age group reported in the MMWR for 1980 (1).

According to current recommendations of both the American Academy of Pediatrics (AAP) and the Immunization Practices Advisory Committee (ACIP), an infant should receive 3 doses of diphtheria-tetanus-pertussis (DTP) vaccine by 6 months of age and a fourth dose by 18 months of age in the absence of medical contraindications (2,3). Of 479 reported pertussis patients, ages 6 months-9 years,* with known vaccination status, 33% were not vaccinated with DTP before becoming ill, and 60% had received 3 doses.

Findings for the patients on whom clinical information was available showed that 72% had a whoop and 41% had apnea. Pneumonia was reported as a complication for 29% of patients (Table 1). The frequency of pneumonia was highest (34%) among infants 6 months of age. For patients 4 years of age, the likelihood of pneumonia was inversely related to the number of DTP doses (for children 0-5 months, p = 0.009; for children 6 months-4 years, p = 0.025.)**

Seizures were reported for 51 (4%) of 1,277 patients; 29 (56%) of these patients were 6 months of age. Among patients 4 years of age, the likelihood of developing seizures was also inversely related to the number of DTP doses received; however, the difference was not statistically significant. Encephalopathy was associated with 0.4% of cases. All patients reported to have encephalopathy were 1 year of age.

Fifty-eight percent of all reported patients were hospitalized, including 80% of infants 6 months of age, 60% of those 6-11 months of age, and 35% of those 1-4 years of age. Relatively few patients 4 years of age were hospitalized. The proportion of patients hospitalized was inversely related to the number of doses of DTP vaccine received; the highest proportion of patients hospitalized occurred among children who had received 3 doses of DTP vaccine (p = 0.0046 for 6- to 11-month olds and 0.0001 for 1- to 4-year olds).

Seven deaths (0.5%) associated with pertussis were reported. All deaths occurred among hospitalized patients who were 1 year of age, had not been vaccinated, and had pneumonia.

Laboratory confirmation of the diagnosis of pertussis was available for 72% of cases. The diagnosis was confirmed by direct fluorescent antibody (DFA) testing in 46% of cases, by DFA and culture in 18%, and by culture in 8%. The diagnosis was made on clinical grounds alone in 28% of cases.

Additional information about household contacts was available for 287 patients. To calculate vaccine efficacy, secondary attack rates were determined for unvaccinated household contacts (no DTP doses received) and for household contacts who were fully vaccinated (3 or more DTP doses). Vaccine efficacy for household contacts 5 years of age was 82.4%. Efficacy could not be calculated for children 5-9 years of age because very few household contacts in this age group were unvaccinated. Reported by Respiratory and Special Pathogens Br, Div of Bacterial Diseases, Center for Infectious Diseases, Surveillance, Investigations and Research Br, Immunization Div, Center for Prevention Svcs, CDC.

Editorial Note

Editorial Note: During the recent controversy regarding pertussis vaccine, questions were raised as to whether the disease has caused sufficient morbidity and mortality, in recent years, to justify routine vaccine use. As with other vaccines, the decision to recommend use of DTP vaccine depends on the risk of developing disease, disease severity, vaccine efficacy, and adverse reactions associated with the vaccine.

Data from the CDC pertussis surveillance systems presented above provide information on the current morbidity and mortality due to pertussis in the United States and on the efficacy of currently used vaccines in both preventing and attenuating disease. They demonstrate that pertussis is a severe disease, particularly for children 1 year of age, and may be associated with seizures, encephalopathy, and death. The data also demonstrate that DTP vaccine is efficacious. More than 80% of children exposed to pertussis who have received at least 3 doses of DTP vaccine will be protected. Because, under the current ACIP vaccination schedule, children do not receive 3 doses of vaccine until 6 months of age, not all cases of pertussis in this age group are preventable. However, children who have received 1 or 2 doses of vaccine, if infected, tend to have milder illnesses than unvaccinated children of similar age. Furthermore, the risk of infection in the 6-month age group may also be reduced indirectly by high levels of vaccination in older children, and the resultant decreased transmission of disease.

As with most surveillance systems, underreporting is a problem. The supplementary system reported only 20% as many cases as were reported to the MMWR in 1979-1981. A disproportionate number of hospitalized, laboratory-confirmed, and classical cases may have been reported. Furthermore, a history of hospitalization may not indicate the same degree of disease severity for different age groups. Nevertheless, the data are useful for estimating the risks of disease and the benefits of vaccine usage.

Two recent studies provide estimates of risks associated with DTP vaccination. Of 15,752 recipients of DTP vaccine, 64% reported local reactions, and 50% reported minor systemic reactions within 48 hours (4). The more serious reactions, such as convulsions noted in 9 children and hypotonic hyporesponsive episodes noted in 9 children, each occurred at a frequency of 1/1,750 doses. Seventeen of the 18 children who had such reactions were examined by a physician shortly after the episode; all were found to be normal.

In Great Britain, 1,000 cases of neurologic illness were investigated by means of a case-control study to detect the occurrence of severe neurologic reactions following DTP vaccination (5). Neurologic illness attributable to DTP vaccination was estimated to occur at a frequency of 1/110,000 doses of DTP, and permanent neurologic residua to occur at a frequency of 1/310,000 doses.

In 1980, 95% of children in the United States had completed a primary series of vaccination by the time they entered school. Because of high levels of vaccine acceptance in the United States, the current risk of pertussis is low. However, the agent, Bordetella pertussis, continues to cause disease. Recent experience in Japan and Great Britain has demonstrated that a decline in vaccination level in a previously highly vaccinated population may result in resurgence of disease (6,7). In addition to the data presented, formal cost-benefit analysis of pertussis and DTP vaccine also indicates that the benefits of vaccine continue to outweigh the risks (8). Both the ACIP and the AAP continue to recommend routine use of DTP vaccine.

References

1. CDC. Annual Summary, 1980. MMWR 1981;29:10.

2. American Academy of Pediatrics. Report of the Committee on Infectious Diseases, 18th ed. Evanston, Illinois: American Academy of Pediatrics, 1977.

3. CDC. Diphtheria, tetanus, and pertussis: guidelines for vaccine prophylaxis and other preventive measures. MMWR 1981;30:393-407.

4. Cody CL, Baraff LJ, Cherry JD, Marcy SM, Manclark CR. Nature and rates of adverse reactions associated with DTP and DT immunizations in infants and children. Pediatrics 1981;68:650-60.

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

6. Bureau of Biologics. Food and Drug Administration. New pertussis vaccine--laboratory and clinical evaluation.(Workshop) Bethesda, Md.:National Institute of Allergy and Infectious Disease, National Institutes of Health, February 11-12, 1982.

7. Joint Committee on Vaccination and Immunization. The whooping cough epidemic 1977-1979. Report. London: Her Majesty's Stationery Office, 1981.

8. Koplan JP, Schoenbaum SC, Weinstein MC, Fraser DW. Pertussis vaccine--an analysis of benefits, risks and costs. N Engl J Med 1979;301:906-11. *Vaccination histories for persons 9 years old were thought to be less reliable and were therefore not included in this analysis. **Chi square test for linear trend, used for all statistical analyses.

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