Orginal Article

Using an influenza epidemic threshold different from those in the United States and Europe caused longer epidemic periods in Korea during the 2018–2019, 2019–2020, and 2022–2023 seasons: a comparative study

Joowon Lee*, Sooyoung Huh, Haesook Seo
Author Information & Copyright
1Infectious Disease Research Center, Seoul Metropolitan Government, Seoul 04524, Korea.
*Corresponding Author: Joowon Lee, Phone: +82-2-2133-9654. E-mail:

© Copyright 2024 Ewha Womans University School of Medicine. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received: Jan 26, 2024; Revised: Feb 16, 2024; Accepted: Mar 07, 2024

Published Online: Apr 30, 2024


Background: Seasonal influenza is an acute respiratory infection caused by influenza virus, which circulates from winter to spring every year in Korea. Influenza surveillance system has been in place to minimize socioeconomic impact of influenza epidemics. The start of influenza season is declared when an observed weekly influenza-like illness value exceeds an epidemic threshold for that season, which is calculated using data from the most recent three influenza seasons. After COVID-19 pandemic, the first seasonal influenza epidemic was declared in the 37<sup>th</sup> week of 2022 and has continued to winter in 2023-24, but this finding has not been observed in the United States and Europe. Methods: Korea, the United States and Europe adopt different methods to set seasonal influenza epidemic thresholds. First, we calculated influenza epidemic thresholds for domestic influenza seasons using different methods of those three regions. Then, we compared the duration of influenza epidemic periods for the most recent three influenza seasons using the three different epidemic thresholds. Results: Epidemic thresholds calculated by Korean method were estimated lower than those calculated by the other methods, and durations of epidemics defined by Korean thresholds were estimated longer than those defined by the other thresholds. Conclusion: The long influenza epidemic that was declared in 2022 and continues to the present could be a wrong interpretation of the phenomenon which is a combination of a low epidemic threshold and increased activities of other respiratory viruses. A more reliable epidemic threshold for seasonal influenza surveillance would need to be established.

Keywords: Influenza, Human; Sentinel Surveillance; Epidemics; Seasons