Investigation of Influenza Outbreak in Healthcare Workers, Hospital A, Buriram Province, Thailand, August – October 2010

Abstract

Background: On 6 October 2010, the Bureau of Epidemiology was notified of influenza-like illness (ILI) outbreak in healthcare workers in ER and OPD of Hospital A, Buriram Province. An investigation was conducted to confirm a diagnosis, identify risks factors of ILI and evaluate the effectiveness of the 2010 trivalent influenza vaccine in this outbreak.
Method: Influenza surveillance reports and medical records were reviewed. Active case finding was conducted in all healthcare workers. An acute respiratory illness (ARI) case was defined as a person who worked in Hospital A and developed at least two of the following symptoms: fever, cough, rhinorrhea and sore throat between 12 August and 31 October 2010. A confirmed influenza case was an ARI case who had a throat swab positive for influenza by RT-PCR. A retrospective cohort study and an environmental survey were carried out in ER and OPD. An ILI case was a person who worked in ER and OPD of Hospital A and developed fever with cough and/or sore throat between 12 August and 31 October 2010. Risk factors for ILI were analyzed in univariate and multivariate analyses and presented by using crude risk ratio (RR) and adjusted odds ratio (OR) with 95% confidence interval (95%CI).
Results: Of 390 healthcare workers in Hospital A, 137 (35.2%) met the case definitions, including 134 ARI cases and 3 confirmed influenza cases (i.e., a case of influenza A H1N1 2009 case and 2 cases of influenza A H3N2). The median age of cases was 32 years (rang: 20 - 60 years) and male to female ratio was 1:5. The highest attack rate was observed in OPD (59.1%), followed by Pharmacy Department (55.2%) and ER (45.0%). The index case worked in Pharmacy Department and she developed symptoms after having close contacts with her son whom was diagnosed with pneumonia. The durations of viral shedding time in two H3N2 influenza cases were 8 and 9 days in the 2 vaccinated nurses. Significant protective factor was hand washing (adjusted odds ratio = 0.161, 95% CI = 0.034, 0.748). The effectiveness of the 2010 trivalent influenza vaccine for preventing ILI was 41.6 % (95% CI = 7.1%, 63.3%). Environmental survey revealed close contact of hospital personnel in a crowded condition and air conditioning area in the hospital.
Conclusion: An outbreak of laboratory - confirmed influenza A H1N1 2009 and influenza A H3N2 occurred among healthcare workers in a tertiary care hospital. The highest attack rates were in OPD, ER and Pharmacy Department. Hand washing was a significant protective factor while the effectiveness of the 2010 trivalent influenza vaccine was modest.