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Influenza is a highly contagious disease. Due to a high number of reported cases with influenza-like illnesses at a prison, a joint investigation by the Department of Disease Control and local public health teams was conducted to confirm the reports and implement control measures. Suspected influenza cases were defined as a prison inmate or officer who developed fever with coughing and/or a sore throat. A confirmed case was a suspected case with a nasopharyngeal swab testing positive for influenza by RT-PCR. The prison environment and health practices were observed. A retrospective cohort study was conducted among prisoners to determine factors associated with influenza diagnosis. Pearson’s correlation between the attack rate in each area and inmate density was calculated. The overall attack rate was 12.7% (326/346) and 16 out of 19 tested positive for influenza A (H1N1) pandemic 2009. Vaccine effectiveness among prisoners who had history of influenza vaccination more than or equal to 2 weeks and less than 2 weeks prior to the outbreak was 25.9% and 13.8%, respectively. Sleeping near a case (adjusted odds ratio (OR) = 3.60, 95% confidence interval (CI) = 1.62-8.00), working near cases (adjusted OR=3.39, 95% CI: 1.59-7.22), and sharing cigarettes with other cases (adjusted OR=2.46, 95% CI: 1.15-4.56) were significant risk factors. A strong correlation between attack rate and area density was found (r=0.68, P-value=0.025). Rapid transmission and high attack rates were probably attributable to overcrowded conditions. Expanded provision of vaccination in prisons should be implemented.
Keywords: influenza A (H1N1) 2009, influenza, vaccine effectiveness, prison, inmate
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