Outbreak investigation of pertussis in an elementary school: A case-control study among vaccinated students

Purpose: A pertussis patient from an elementary school, in Gyeonggi Province, Korea, was notified to public health authority on July 25, 2017. Epidemiologic investigation was conducted to identify the magnitude, possible source of infection and risk factors for this outbreak on August 17, 2017. Materials and Methods: A case was defined as the school student experiencing cough for more than two weeks with or without paroxysmal, whoop, or post-tussive vomiting. Control was defined as the student polymerase chain reaction‒negative at the school. School based surveillance was implemented to identify additional cases. Results: From June 29 to August 27, 2017, nine patients of pertussis were identified from an elementary school. Among nine cases, eight were confirmed by polymerase chain reaction positive. All cases had cough, one (11%) had post-tussive vomiting, and one (11%) had fever. Eight cases had macrolide for 7 days in outpatient clinic, and one case admitted in a hospital. There was no significant difference of demographic factors including gender (p =0.49), age group (p =0.97), number of series of vaccination of pertussis (p=0.52), the number of participation of after school activity (p=0.28), and the time elapsed since last vaccination (p=0.42). However, we found the history of contact within the classroom or after-school activity was only the independent risk factor among all the demographic factors collected (odds ratio, 63.61; 95% confidence interval, 4.35 to 930.79). Conclusion: The contributing factor for transmission is associated with the case-contact. Immediate identification of pertussis with use of appropriate diagnostic test may help to avoid a large number of cases.