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BACKGROUND: Nationwide, the incidence of pertussis is greatest among infants, adolescents, and children aged seven to ten years. Waning immunity to pertussis following vaccination has been observed in adolescents and children aged five to seven years. In September 2013 the Florida Department of Health (DOH) was notified of a positive polymerase chain reaction test for pertussis in a one year old vaccine-exempt preschool attendee. Appropriate containment measures were instituted for the household and classroom. Despite the interventions, six confirmed cases from two households linked to the preschool were identified in December. Following the identification of pertussis in a third household, DOH made a site visit to the preschool to investigate pertussis transmission and vaccination status among preschool attendees and staff.
METHODS: Preschool staff were asked to identify any students and staff with cough illness. A standardized case report form was used to collect data. Parents were asked to complete household cough illness questionnaires (98% completion rate). Cases were classified using the national case definition for pertussis. Cases were analyzed by age, classroom, vaccination status, and disease classification.
RESULTS: Eleven cases from September 2013 through January 2014 were classified as confirmed: five laboratory-confirmed and six epi-linked household contacts. Twenty-eight cases were classified as probable. Four cases were hospitalized. Twenty-six students aged one to five years (attack rate = 22%) and two staff (attack rate = 7%) were identified as cases. The remaining eleven cases were linked to the preschool. Attack rates nearing 50% were identified among 17 vaccinated children in the one classroom with a staff member with laboratory-confirmed pertussis. Only five out of 117 children in the preschool were not current on pertussis vaccinations, of which two were cases. Of the 33 child cases identified, 28 had received three or more acellular pertussis vaccinations, and 22 had received four or more. The average number of days from last vaccination to onset of symptoms for the children was only 624 days and eleven (33%) children were vaccinated within the past year.
CONCLUSIONS: Sustained transmission of pertussis in a highly vaccinated cohort of one to five year olds has not been previously identified. Many physicians were hesitant to diagnosis and did not test for pertussis given the recent vaccination history of the patients, despite notification of an ongoing laboratory-confirmed pertussis outbreak. Recent pertussis vaccination should not preclude diagnosis, testing, and treatment of presumptive pertussis.