Online anti-vaccine articles contribute to the anti-vaccine movement, which leads to recent outbreaks of vaccine-preventable diseases. Previous studies indicate that anti-vaccine articles are easy to read and understand, which may increase their abilities to engage viewers. The present study aims to examine if readability levels are related to engagement. Using combination of terms to search for vaccine articles in Google in May 2017, this study examined 541 pro-vaccine online articles with a total of 508,571 words and 382 anti-vaccine articles with a total of 843,805 words. Almost all vaccine articles exceeded the American average reading comprehension level. No significant difference in readability was found between pro- and anti-vaccine articles. Pro-vaccine articles that could only be understood by college graduates were less engaging than those with lower readability levels. No significant relationship between anti-vaccine articles' readability and engagement was discovered. Different vaccine topics had different readability and engagement levels, which implied that certain combinations of themes and readability levels could enhance the health messages' persuasion effect. Recommendations for designing effective health messages are provided.