A 5-month pertussis outbreak in a Florida preschool with a high vaccination rate highlights the need for efforts to reduce transmission and provide booster vaccinations for adults in regular close contact with young children.

The sustained transmission of whooping cough at the school “raises concerns about inadequate protection against pertussis in an age group believed to be well protected by acellular pertussis vaccination,” write public health researchers led by James Matthias, MPH, an epidemiology and surveillance unit manager with the Florida Department of Health in Tallahassee.

The spate of cases, detailed in an article published online January 13 in Emerging Infectious Diseases, began in a 1-year-old preschooler and eventually affected 26 of 117 pupils, 2 adult employees, and 11 family members or contacts.

The school’s pupils, aged 10 months to 6 years, were all age-eligible for at least three doses of the diphtheria/tetanus/pertussis vaccine, and only five children, including two whooping cough cases, had not received the complete series.

Alarmingly, the highest case rate, 48%, occurred in one class where all 17 pupils had received the full series. That raises red flags “about vaccine effectiveness in this preschool age group and reinforces the idea that recent pertussis vaccination should not dissuade physicians from diagnosing, testing, or treating persons with compatible illness for pertussis,” Matthias and colleagues write. The classroom’s teacher, who had laboratory-confirmed pertussis, continued to work while ill and had not received a booster shot.

According to the investigation, many physicians seemed to have been reluctant to test for and diagnose pertussis in light of patients’ recent immunization histories. “Hesitation by providers in reporting presumptive pertussis delays public health response to prevent continued transmission of pertussis in the community,” the authors write. “[R]ecent pertussis vaccination should not preclude diagnosis, testing, and treatment of presumptive pertussis cases.”

They note a spike in reported US cases of whooping cough from 2000 to 2012, with a more than sixfold jump from 7867 to 48,277. One possible factor may be waning immunity. “Reports of genetic changes in circulating [Bordetella] pertussis have raised concern that this organism could be adapting to vaccine-induced immunity,” the authors write. They suggest that acellular pertussis vaccine performance in preschoolers be monitored to ascertain whether the Tallahassee outbreak was an isolated event or part of an emerging epidemiologic trend.

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