Abrupt rebound of seasonal Influenza A [H3NX] epidemic following the COVID-19 outbreak in a prison, Phitsanulok, Thailand

Abstract

Background: On 21st October 2021, Office of Disease Prevention and Control Region 2 Phitsanulok, was notified of 200 suspected influenza cases in a prison in Phitsanulok. This prison used to have a large influenza A outbreak in 2018 and a COVID-19 outbreak a month before this notification. The outbreak investigation was conducted to verify the diagnosis and the outbreak, describe epidemiological characteristics, identify events, activities and behaviors potentially related with the outbreak, point out any changes following two previous outbreaks, and implement prevention and control measures.
Methods: We conducted descriptive study by reviewing early COVID-19 situation and performing active case finding for suspected influenza cases in the prison. Meanwhile, we reviewed medical records of severe and dead cases. Nasopharyngeal and throat swabs were collected for Real–time RT–PCR for respiratory viruses. Furthermore, we interviewed officers and prisoner health volunteers about any suspicious activities and control measure changes following two previous outbreaks. We also observed inmates’ behavior.
Results: Both number of COVID-19 cases and the laboratory positive rate had declined before presenting of suspected influenza cases. We found 280 male inmates that met case definition (attack rate = 11.3%), and 13 were sampled to submit specimens for laboratory testing. All of them were positive for Influenza A (H3NX) and negative for SARS-CoV-2. Six cases were severe and one was dead. The median age of cases was 31 years (Interquartile range: 13 years). Cases were scattered in every zone while the outbreak began on 16th October 2022 and peaked on 20th October 2022, approximately a month after the peak of COVID-19. Gathering events were suspected to be the cause of spreading. Following several outbreaks, all inmates had their own drinking glass and wore face mask. The prison also invested in data management, virtual visit application and telemedicine.
Conclusion and Recommendation: An outbreak of 280 Influenza A (H3NX) inmates occurred in the prison right after the COVID-19 outbreak. Gathering events were the potential cause of spreading. Despite significant improvement after two large respiratory disease outbreaks, the prison should improve screening and isolation strategies using case scenario exercise.