Associated Factors of Influenza B Infection from a Staff-introduced Outbreak in a Prison, Buriram Province, Thailand, September–November 2023

On 17 Oct 2023, the Office of Disease Prevention and Control Region 9 Nakhon Ratchasima was notified of 570 influenza-like illness cases in a prison in Buriram Province, Thailand. An investigation was performed to confirm the outbreak and diagnosis, identify sources and risks, and provide recommendations. National surveillance database and prison records were reviewed, and an active case-finding in the prison targeted individuals with fever, cough, sore throat, runny nose, or myalgia between 20 Sep 2023 and 18 Nov 2023. Suspected cases were individuals who developed fever with at least one other symptom. Confirmed cases were suspected cases with a positive result based on the reverse transcription polymerase chain reaction technique. A total of 678 cases (attack rate 39.6%) were identified, including 28 confirmed influenza B cases. Three were admitted and there were no intubations or deaths. Vaccine coverage in the prison was 33.1%. Observations revealed overcrowding, poor ventilation, inadequate sick-person management, and a lack of hygiene facilities. A case-control study considering suspected and confirmed cases as true cases and symptom-free inmates as true non-cases identified exposure to close-range sneezing (odds ratio 2.03, 95% confidence interval 1.23–3.36) and sporadic face mask-wearing (odds ratio 1.67, 95% confidence interval 1.07–2.63) as significant risk factors after adjusting for other factors. Interventions included screening, early quarantine, event cancelation, and enhanced hygiene practices. The outbreak subsided within three weeks. We recommended the use of prison records to develop a surveillance system, improving sick staff management, increasing vaccine coverage, and reducing overcrowding.