An outbreak investigation of influenza A (H1N1) in a prison, Phetchabun Province, Thailand, May-July 2024

Abstract

On June 21, 2024, Phetchabun Hospital was notified of an influenza outbreak among approximately 50 inmates at a local prison. The hospital’s Surveillance and Rapid Response Team ( SRRT), in collaboration with relevant agencies, conducted an investigation from June 22 to 24, 2024, aiming to confirm the outbreak and diagnosis, describe the epidemiological characteristics, identify risk factors, and propose control measures. The investigation comprised a descriptive epidemiological study, laboratory testing, an environmental survey, and an analytical component using a retrospective case-control design. Data were analyzed using logistic regression. A total of 416 cases were identified, with a 26.5% attack rate, and the most common symptoms were fever and cough. The outbreak, initially affecting male inmates, subsequently spread to female inmates in a propagative pattern. Associated risk factors included sharing drinking glasses and towels (adjusted OR=3.65, 95% CI: 1.77-7.48, p-value<0.001 and adjusted OR=1.97, 95% CI: 1-3.87, p<0.05), while receiving the 2024 seasonal flu vaccine was a protective factor (adjusted OR=0.16, 95% CI: 0.07-0.38, p<0.05). The outbreak was caused by an H1N1 influenza strain, likely introduced by new inmates. Screening and isolating new inmates from existing ones are essential measures to prevent future outbreaks. Additionally, administering annual influenza vaccination to inmates is an essential measure that must be implemented to help mitigate disease severity and reduce the risk of outbreaks.