  It has been one year since the outbreak of the COVID-19 pandemic. The good
news is that vaccines developed by several manufacturers are being actively
distributed worldwide. However, as more and more vaccines become available to
the public, various concerns related to vaccines become the primary barriers
that may hinder the public from getting vaccinated. Considering the
complexities of these concerns and their potential hazards, this study aims to
offer a clear understanding about different population groups' underlying
concerns when they talk about COVID-19 vaccines, particular those active on
Reddit. The goal is achieved by applying LDA and LIWC to characterizing the
pertaining discourse with insights generated through a combination of
quantitative and qualitative comparisons. Findings include: 1) during the
pandemic, the proportion of Reddit comments predominated by conspiracy theories
outweighed that of any other topics; 2) each subreddit has its own user bases,
so information posted in one subreddit may not reach those from other
subreddits; 3) since users' concerns vary across time and subreddits,
communication strategies must be adjusted according to specific needs. The
results of this study manifest challenges as well as opportunities in the
process of designing effective communication and immunization programs.
