Outbreak Investigation of Varicella (Chickenpox) in a Junior High School, Ban Kaeng Subdistrict, Si Satchanalai District, Sukhothai Province.

Abstract

On January 26, 2026, the Situation Awareness Team (SAT) of the Sukhothai Provincial Public Health Office, in collaboration with the Sri Satchanalai District Surveillance and Rapid Response Team (SRRT), investigated a cluster of chickenpox cases at a school in Ban Kaeng Subdistrict, Sri Satchanalai District, Sukhothai Province. The objectives were to confirm the diagnosis and the outbreak, describe the epidemiological characteristics, identify risk factors, and recommend appropriate control measures. A descriptive study and an analytical case-control study were conducted. Active case finding was performed in the school. A case was defined as any student or staff member who developed a rash, vesicles, or crusted lesions between December 21, 2025, and March 9, 2026. The diagnosis was confirmed based on clinical manifestations, and the outbreak was confirmed by the occurrence of more than two clustered cases in the same setting. A total of 28 cases were identified among 239 students and staff members, yielding an attack rate of 11.72%. The median age was 10 years (IQR = 10). The most common symptom was vesicles (89.29%). The index case was a 33-year-old male teacher who taught Grade 4 and developed symptoms on January 5, 2026. Risk factor analysis showed that the factors significantly associated with the outbreak included close contact during activities (adjusted OR = 35.01, 95% CI = 13.08–93.66), epidemiological linkage to the source of infection (adjusted OR = 22.11, 95% CI = 8.56–57.00), and environmental conditions within classroom buildings (adjusted OR = 4.35, 95% CI = 1.82–10.34) were significantly associated with the chickenpox outbreak. Recommended control measures included strict daily morning symptom screening, isolation of cases until all lesions had crusted, improvement of environmental sanitation, and enhancement of ventilation within classroom buildings to reduce crowding and the risk of exposure.