The 1993 epidemic of pertussis in Cincinnati. Resurgence of disease in a highly immunized population of children.

BACKGROUND

In 1993 there was a resurgence of pertussis in the United States. Altogether, 6335 cases were reported, the most in 26 years.

METHODS

Using active microbiologic surveillance, we investigated the epidemic of pertussis in Greater Cincinnati in 1993. The population of 1.7 million in this area is served by a single children's hospital and pertussis laboratory. We prospectively followed patients given a new diagnosis of pertussis in July through September 1993 to determine the characteristics of the epidemic.

RESULTS

From 1979 to 1992, there was a cumulative total of 542 cases of pertussis. In 1993, 352 cases were diagnosed, an increase of 259 percent over the 1992 total. Sixty-three percent of the cases had positive cultures for Bordetella pertussis, 18 percent were positive on direct fluorescent-antibody testing only, and 19 percent were diagnosed clinically. The outbreak began in the suburbs during the summer and spread through Greater Cincinnati. Of 255 total cases diagnosed in July through September (195 excess cases over the maximal base-line level of 20 per month in the previous 14 years), 75 percent were in white patients and 67 percent of the patients had private insurance or paid for care out of pocket. In 1993, as compared with 1979 through 1992, there was a shift in incidence from younger infants to older children; the percentages of cases according to age group were as follows: 0 to 6 months, 53 percent from 1979 through 1992 and 35 percent in 1993 (P < 0.001); 7 months to 5 years, 33 percent and 43 percent (P < 0.002); 6 to 12 years, 5 percent and 11 percent (P < 0.001); and more than 12 years, 5 percent and 11 percent (P < 0.003). Immunization records revealed that 74 percent (75 of 101) of the children with pertussis who were 19 months to 12 years old had received four or five doses of the combined diphtheria-pertussis-tetanus (DPT) vaccine, and that 82 percent (103 of 126) of those 7 to 71 months old had received at least three doses of DPT vaccine. The whole-cell vaccines used came from both of the major manufacturers (Connaught Laboratories and Lederle Laboratories). Disease was not severe, but 80 of the 255 children (31 percent) given diagnoses during the three epidemic months were hospitalized. There were no deaths.

CONCLUSIONS

Since the 1993 pertussis epidemic in Cincinnati occurred primarily among children who had been appropriately immunized, it is clear that the whole-cell pertussis vaccine failed to give full protection against the disease.