Sudden pertussis outbreaks in Guro-gu, Seoul: epidemiological characteristics and contributing factors in 2024.

PURPOSE: Although pertussis is a vaccine-preventable disease, its incidence increased rapidly in Guro-gu, Seoul, since May 2024. This study aimed to investigate the epidemiological characteristics of pertussis cases and outbreaks during this period. METHODS: Data from epidemiologic investigations of 355 laboratory-confirmed pertussis cases reported in 2024 in Guro-gu, Seoul, were analyzed using national surveillance records. Demographics, vaccination status, outbreak-associated educational settings, and symptom-to-test intervals were analyzed using descriptive statistics and non-parametric tests. RESULTS: Between May and December 2024, 355 cases were reported, 85.4% of which occurred among school-aged children and adolescents, representing a marked increase compared with previous years. Outbreaks were identified in two elementary schools, three middle schools, and four private academies, indicating clustered transmission in educational settings, with school-based attack rates ranging from 3.2% to 6.3%. Among 146 outbreak-associated school cases, 95.4% had completed age-appropriate vaccination on time. The symptom-to-test interval showed a right-skewed distribution (median, 6 days; interquartile range, 3-11), with longer delays observed in high school students. CONCLUSION: The sharp increase in pertussis cases among school-aged children underscores the need for strengthened surveillance in educational settings and improved timeliness of testing to reduce transmission. Delays in symptom-to-test intervals and the occurrence of cases despite age-appropriate vaccination suggest possible waning immunity, highlighting the importance of evaluating booster vaccination strategies and targeted age-specific public health interventions.