An outbreak of influenza a H3N2 in a prison, Krabi Province, Thailand, August, 2023

Abstract

The outbreak of influenza tends to increase every year, especially in prisons where several outbreaks have occurred. On 15 August 2023, a joint investigation team received a notification that there was a cluster of influenza-like illness (ILI) cases more than 100 cases in a prison, Krabi province. We investigated the outbreak to confirm the diagnosis and its epidemiology, to evaluate the magnitude of outbreak, to identify causes and mode of transmission, and to implement the effective control measures. A descriptive study was conducted and active case finding was performed. Suspected cases were defined as any prisoner or staff member of the prison who had at least 2 of the following symptoms: fever, cough, sore throat, runny nose, headache and muscle pain during 7-19 August 2018. We collected the samples for real-time RT-PCR and whole genome sequencing, and calculated the capacity of the prison cell. Total of 325 cases were met case definition (attack rate 26.1%): 35 of 39 probable cases (89.7%) were confirmed positive for influenza A H3N2 by RT-PCR. Whole genome sequencing analysis revealed that all 7 cases were genetically linked with a substrain called A/Darwin/9/2021(H3N2)-like strain. Male prisoners had the highest incidence rate at 29.2%. The effectiveness of the influenza vaccine against clinical influenza was 39.4%. The outbreak began when a staff member got sick and spread to the prisoners. Risk behaviors that lead to disease transmission were identified as staying close to patients (76.6%), doing activities together with patients (70.5%), and sharing items with patients (44.0%). The prison cells have a capacity rate below the standard (area<1.6 sq.m/person) at 47.4%. We implemented the measures to stop the transmission of the disease, which included screened for new cases every day and immediately isolated patients, stopped activities in the prison, stopped family visits, increased the frequency of cleaning, encouraged to wear masks, and immediately prescribed oseltamivir for the patients.