The present study investigated students' (N = 404) interpretations of the main message and use of modes in a persuasive multimodal video on vaccines. It also examined whether students' topic knowledge, language arts grades, and self-identified gender were associated with their interpretations. Students analyzed a YouTube video in which two entertainers demonstrated the importance of vaccinating children. Students' interpretations of the usefulness of vaccines varied in terms of quality of reasoning, which was associated with students' topic knowledge. Notably, many students' interpretations of the use of modes were incomplete, or they did not even mention certain modes in their response. The results suggest that students should be explicitly taught how to interpret different modes and their uses for argumentative purposes.