# Epidemiologic Investigation of a Varicella Outbreak in an Elementary School in Gyeonggi Province, Republic of Korea

## TL;DR

An outbreak of chickenpox in a highly vaccinated school in South Korea was studied to understand how it spread and what factors contributed to it.

## Contribution

The study highlights the role of private academies in amplifying chickenpox transmission despite high vaccination rates.

## Key findings

- 25.8% of 89 contacts developed varicella, with a mean incubation period of 13 days.
- Outbreak originated from a single case and spread via households and private academies.
- High vaccination coverage did not prevent transmission due to waning immunity.

## Abstract

Background/Objectives: On 6 June 2023, two varicella cases were reported at a highly vaccinated elementary school in Gyeonggi Province, Republic of Korea. We investigated the outbreak to describe its transmission dynamics; quantify attack rates in school, household, and private-academy settings; and assess the impact of coordinated control measures. Methods: A case-series study included 89 teachers and students who had contact with suspected patients. Using case definitions, laboratory tests, questionnaires, and environmental assessments, we evaluated exposures and factors facilitating spread. Results: Varicella developed in 23 of 89 contacts (25.8%); laboratory confirmation was obtained in 2 (8.7% of cases). The mean incubation period was 13 days. Epidemic-curve and network analyses indicated that the outbreak began with a single index case and extended through household contacts and private educational facilities, ultimately involving multiple schools. Conclusions: Breakthrough transmission can occur even when single-dose coverage exceeds 95%, particularly as vaccine-induced immunity may wane over time. Poorly regulated extracurricular facilities, such as private academies, act as bridging hubs that amplify spread across grades and even between schools. For timely detection and control, these venues should be incorporated into routine varicella surveillance, and rapid, coordinated infection-control measures are required across all educational settings.