AIRFLOW, BEHAVIORS AND DELAYED DETECTION: DRIVERS OF A COVID-19 OUTBREAK IN A THAI MILITARY RECRUIT TRAINING UNIT, MAY-JUNE 2025

Background: On May 6, 2025, the Epidemiology Unit of Phramongkutklao Hospital was notified of a probable COVID-19 outbreak among recruits and instructors at a recruit training unit in Samut Sakhon Province, Thailand. A joint field investigation was conducted in collaboration with the 11th Medical Unit, the Epidemiology Unit of Phramongkutklao Hospital, and the Armed Forces Research Institute of Medical Sciences (AFRIMS). Objective: To confirm and characterize the COVID-19 outbreak, identify critical gaps in the existing protocol and transmission factors, and provide recommendations for control and prevention. Methods: A retrospective cohort study was conducted from May 1 to June 3, 2025. Cases included individuals with at least two COVID-19–compatible symptoms, a positive antigen test kit (ATK) result, or laboratory confirmation by real-time RT-PCR. Structured interviews and standardized questionnaires collected exposure and behavior data. A walk-through survey assessed environmental conditions, including airflow, temperature, and humidity in the dormitory. Outbreak control measures, including cohorting based on ATK results, were documented. Results: Among 114 personnel, 47 cases were identified (attack rate 41.2%), with 97.9% symptomatic. Of these, 95.7% had received prior COVID-19 vaccination. Common symptoms were sore throat, headache, and cough. The environmental assessment showed elevated temperatures, humidity, and west-to-east airflow, which correlated with higher attack rates in the eastern sleeping zones. Behavioral factors included shared drinking glasses and proximity during meals; sharing glasses showed the lowest p-value (adjusted OR = 1.99, 95% CI: 0.89–4.48). Delayed detection due to a lack of systematic screening facilitated transmission. Conclusions: The outbreak was driven by multifactorial causes, including delayed detection and environmental and behavioral factors. Alignment between airflow and attack rates highlights the role of ventilation. Proactive surveillance, systematic ATK screening for all respiratory symptoms, and enforcement of physical distancing are recommended to prevent similar outbreaks in congregate military settings.