JMIR Publications

ABSTRACT

Background:

Non-pharmaceutical interventions (NPIs) such as mask-wearing and social distancing during the COVID-19 pandemic significantly reduced the transmission of respiratory viruses. The "immunity debt" theory suggests that decreased exposure to pathogens during this period may have led to increased susceptibility and severity of these viruses once restrictions were lifted. However, the post-pandemic impact of NPIs on the burden of these viruses, particularly in young children, remains underexplored.

Objective:

This study aimed to evaluate the impact of NPIs during the COVID-19 pandemic on the epidemiology and clinical burden of influenza and RSV in Korea, focusing on changes in incidence, timing, and severity of infections in children.

Methods:

We analyzed weekly virologic, outpatient, and inpatient surveillance data for influenza and RSV from the Korea Disease Control and Prevention Agency (KDCA) from 2017 to 2024, covering pre-pandemic, pandemic, and post-pandemic periods. Time-series analyses were conducted to assess trends in incidence and clinical severity across age groups before and after the pandemic.

Results:

Pre-pandemic seasons showed typical winter peaks for both influenza and RSV, with RSV epidemics occurring slightly earlier. Post-pandemic, RSV returned in 2021/22 with delayed and prolonged epidemic peaks, whereas influenza did not resurface until the 2022/23 season. The overall burden of both influenza and RSV was reduced in post-pandemic seasons, though the proportion of severe RSV cases increased among children aged 1-6 years, consistent with the "immunity debt" hypothesis. Hospitalization rates for influenza, however, increased in older children aged 7-18 years post-pandemic.

Conclusions:

The patterns of influenza and RSV infections in Korea following the COVID-19 pandemic reveal distinct shifts in timing, severity, and affected age groups. Substantial increases in susceptibility among younger children for RSV and older children for influenza, potentially due to immunity gaps highlight the lasting impacts of pandemic-related disruptions on population immunity and respiratory disease transmission patterns. Ongoing surveillance and targeted public health measures remain critical to understanding and managing seasonal respiratory viruses in a post-pandemic context.