As globalisation advances, an influx of loanwords has been seen in many languages in recent years. Japanese and Korean have similar grammatical features and many English-based loanwords. This study aims to clarify the difference in loanwords in Japanese and Korean adaptation, focusing on substituting alternative native lexicons through COVID-19. First, we collected COVID-19-related news articles in 2020 and extracted COVID-19-related loanwords in Japanese and Korean. Second, we examined the number of loanwords at an initial stage and investigated their changes compared to their alternative native lexicons. Three primary findings emerged from this study: (1) a similar number of loanwords were observed in Japanese and Korean, (2) the two languages had common features in that new words with different meanings or those that do not exist in English (pseudo-anglicisms) were devised based on the adapted loanwords, and (3) more loanwords were retained in Japanese, while relatively many loanwords in Korean became extinct or were replaced by loanwords' alternative native lexicons with time. These findings indicate that substitution by alternative native lexicons leads to the lower usage of loanwords in Korean, even though the two languages adapted similar quantities of loanwords at the initial stage.