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

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^th 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.