The Effect of the Pandemic H1N1 Monovalent Vaccine on the Influenza Outbreak in a Prison, Nakhon Sawan Province, Thailand, August-September 2010

Abstract

Background: On August 29^th, 2010, an outbreak of pandemic influenza A (H1N1) 2009 in a prison was detected. About one half of female prisoners and one third of male prisoners received influenza A (H1N1) 2009 monovalent vaccine a few weeks before the outbreak started. An investigation was conducted to evaluate an effectiveness of the monovalent vaccine.
Methods: We reviewed treatment records and conducted active case finding in the prison. An Influenza-like illness (ILI) case was defined as a person who developed fever with sore throat and/or cough during August 20^th- September 8^th, 2010. A laboratory-confirmed influenza case was identified by either throat swab that positive real-time RT-PCR for influenza or ≥ 4-fold rise of HI titer from paired serum samples. A retrospective cohort study was carried-out to identify risk factors and to estimate effectiveness of the monovalent H1N1 vaccine.
Results: Of 2,482 male prisoners, 216 (8.7%) met ILI case definition and 65 cases (51.6%) were confirmed H1N1 2009 infection. In a cohort of 688 male prisoners, H1N1 vaccination was a significant protective factor (RR=0.52, 95%CI=0.27, 0.98). Infection rates of 50 vaccines and non-vaccinated prisoners were compared and vaccine effectiveness was estimated at 80% (95%CI=57%, 91%). However, the vaccine was 45% (95%CI= -75%, 83%) effective in prevention of ILI symptoms. Only one confirmed case was identified in female prisoners.
Conclusions: H1N1 monolavent vaccine was below 50% effective in prevention of ILI. However, the pandemic vaccine significantly reduced H1N1 2009 infections and resulted in limited transmissions and low attack rate in the prison.