This study examined COVID-19 vaccine videos on TikTok (n = 216 collected in March 2021) during the early days of the vaccine rollout in the U.S., including video source, overall stance toward COVID-19 vaccines, Health Belief Model (HBM)-related content, message features (i.e. humor, video type, message sensation value, on-screen text, and unoriginal sound), and user engagement indices (number of views, shares, comments, and likes). Regarding source, health professionals and general users were two of the main sources, which varied depending on video stance. Pro-vaccine videos occurred the most often from health professionals whereas anti-vaccine videos occurred the most often from general users. Health professionals (vs. general users) generated more views, shares, comments, and likes. Regarding stance, we found more pro- than anti-vaccine videos (57.9% vs. 37.5%). Stance was not related with any user engagement index. Though many videos were pro-vaccine, the content corresponding to HBM-specified factors, which likely facilitate a positive behavioral change, was largely lacking, such as mentions of COVID-19 severity (5.6%), susceptibility (2.8%) and information boosting vaccination self-efficacy (3.7%). Mentions of side effects (34%) emerged as the major vaccination barrier. HBM-related mentions were not related with any user engagement index. COVID-19 vaccine videos used several features, which varied across stance. Pro-vaccine videos featured more musical performance, while anti-vaccine videos used more humor, playacting, sound effects, and unoriginal sound. Several message features (e.g. humor and on-screen text) were positively associated with users' engagement with a video. Practical and theoretical implications of the findings are discussed.