Outbreak investigation of influenza A H3N2 in a primary school, Prakhon Chai, Buriram province, Thailand, 10 July-10 September 2015

Abstract

Background: On 10 September 2015, the Epidemiology center of Buriram Provincial Health Office was notified by Epidemiology center of Prakhon Chai district about an influenza outbreak in primary school in Prakhon Chai district, Buriram province. An investigation was conducted during August 10 - September 10, 2015 with objective to identify risk factor, explain the outbreak and evaluate the prevention and control measure.
Methods: Active case finding was conducted in primary student and school’s personnel. The suspected case was defined as any person who got only fever or at least two of following sign and symptoms: cough, sore throat, rhinorrhea, secretion, dyspnea, fatigue, headache or myalgia. For probable case was defined as any person who got fever with any of following sign and symptoms was described above. A confirmed influenza case was an probable or suspected case who had a throat swab positive for influenza by RT-PCR. A retrospective cohort study and an environmental survey were carried out in school. Risk factors 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 560 attendants, 88 (Attack rate 8.6%) met the case definitions. including 34.1% were suspected cases, 63.6% were probable cases and 2 cases (2.3%) were confirmed influenza A H3N2. The female to male ratio was 1.2:1. The first confirmed case was found on 9 August 2015 and the number of case peaked on 10 August 2015. Close contact with Influenza patients was a significant factor (Adjusted OR=6.17, 95% CI = 3.20, 11.88). There was no significant protective factor in subgroup analysis. School closing and health education was implemented. Incidence of Influenza case decrease rapidly and the last case was reported on 1st September 2015.
Conclusions: Influenza A H3N2 outbreak was confirmed and occurred among attendants. Close contact with Influenza patients was identify as significant risk factor of infection. Decision on intervention measure were effectively to breakdown outbreak circle in the school but there was no evaluation the community outbreak that resulted from those measure.