Understanding the low effectiveness of influenza vaccines in older adults of South Korea: an exploration of contributing factors

Despite achieving high influenza vaccine coverage (>80 %) among older adults, South Korea consistently reports lower influenza vaccine effectiveness (VE) compared to the United States, Canada, and Europe. This viewpoint reviews the potential causes of low VE, highlighting antigenic dissimilarity, immune imprinting, egg-adapted vaccine effects, and low natural exposure. Historical data records frequent antigenic dissimilarities in South Korea, contributing to reduced VE. Additionally, annual vaccination may lead to immune imprinting, potentially limiting effective antibody responses against antigenically distinct strains. Egg-based vaccine production further skews immune responses toward egg-adapted epitopes. In settings with mature vaccination programs, limited natural influenza exposure may paradoxically result in lower observed VE. To address these challenges, we propose strategies including enhanced regional influenza surveillance in global vaccine strain selection, adoption of non-egg-based platforms (mRNA and cell culture vaccines), highly immunogenic vaccines and immuno-focusing techniques. These approaches hold promise for improving future influenza VE in South Korea and globally.