Vaccine hesitancy is a problem attracting growing attention and concern. Communication can be an effective tool to counteract vaccine hesitancy and promote optimal vaccine uptake. Readability has been recognized as one of the more important aspects of health communication for achieving good health literacy. We reviewed studies of readability assessment in the area of vaccine communication. We conducted a systematic literature search in September 2020, using four online databases (Medline, CINAHL, PsycArticles, and PsycINFO). We included studies that assessed the readability level of online and offline vaccine information materials. We found 12 articles that were appropriate for inclusion. Ten of the studies were published after 2016. The readability levels of the majority of the materials assessed were found to be difficult and higher than 8th-grade level. Readability assessments of vaccine information are scarce. The limited evidence shows that the readability level of vaccine information supplied by health care providers is more difficult to read than recommended. More studies on the readability of vaccine information are recommended. Difficulty reading vaccine information may influence attitudes toward acceptance of or hesitancy to take vaccines. It is recommended that health care professionals use guidelines and tools to create easy-to-read vaccine information. •We reviewed the readability of online and offline vaccine information materials.•Only 12 studies were found, indicating readability assessment is scarce in this area.•Reading skill level for vaccine information was much higher than recommended levels.