Risk Factors Associated with an Influenza B Outbreak due to Inefficient Screening in a Prison in Thailand

On 3 Oct 2019, the Office of Disease Prevention and Control Region 12 Songkhla received notification of a suspected influenzacluster in a prison in Songkhla Province, Thailand. In cooperation with the local public health teams, we investigated the event to confirm the diagnosis, identify the source of infection, and implement control measures. A suspected influenza case was defined as an individual with fever and at least one of the following symptoms: cough, sore throat, runny nose, or dyspnea. Aconfirmed case was a suspected case who tested positive to influenza by the reverse transcription polymerase chain reaction technique. An environmental survey was conducted to assess risk behaviors and determine the occupancy rate. A case-control study was performed to identify associated factors of developing influenza. We identified 128 suspected cases, of which seven were confirmed to have type B influenza. All were male and their median (interquartile range) age was 29 years (24–35). The overall attack rate was5.2%. The first case developed symptoms in the new prisoner zone and moved to another zone without undergoing respiratory screening. The analytical study revealed that sleeping within one meter of a confirmed case (adjusted odds ratio (AOR) 1.77, 1.07–2.92), and sharing drinking glasses with others (AOR 1.83, 1.18–2.82)were significant risk factors. The overcrowded prison (165% occupancy) led to limited availability of vaccines, causing 83.5% of prisoners to be unvaccinated. Strict screening of new prisoners before moving to another area and early isolation of symptomatic prisoners are recommended.