An Influenza A (H1N1) Outbreak Investigation in a Prison, Samut Prakan Province, May 2025

Abstract

On 6 May 2025, the Office of Disease Prevention and Control Region 6, Chonburi Province, received a report of an influenza A outbreak in a prison in Samut Prakan Province, with 71 cases testing positive by rapid influenza diagnostic tests. On 7 May 2025, a joint outbreak investigation team conducted a field investigation to confirm the diagnosis and outbreak, describe epidemiological characteristics, identify risk factors, and implement control measures. The investigation included a descriptive epidemiological study, active case finding, laboratory testing, and an environmental assessment, as well as an analytical study using a case–control design. Univariate analysis was performed using the Chi-square test, and variables with p-value < 0.05 were included in multivariable analysis using multiple logistic regression. A total of 92 cases met the case definition (attack rate: 4.7%), with one death reported (case fatality rate: 1.1%). The cases were classified as 11 suspected, 71 probable, and 10 confirmed cases. All cases were male inmates in Zone 7, aged 18-77 years (median age: 37 years). The most common symptoms were fever (100%), cough (67.4%), myalgia (55.4%), and rhinorrhea (54.4%). Significant risk factors associated with illness included not receiving influenza vaccination (adjusted odds ratio [aOR] = 2.17, 95% CI: 1.18–3.98), sharing meals with infected individuals (aOR = 3.20, 95% CI: 1.21–8.48), and inconsistent use of face masks (aOR = 9.31, 95% CI: 2.63–32.89). Contributing factors to the outbreak included the introduction of infection from outside the prison and close-contact group activities. Therefore, appropriate quarantine measures should be implemented for inmates who need to leave the prison, along with strict monitoring of face mask use, physical distancing during group activities, and personal hygiene practices to prevent future outbreaks.