According to Aiyang's inquiry. The comparative of healthy is healthy, which means "healthier". The superlative of healthy is healthy, which means "the healthiest".
For example, you can say:
Heishealthierthanme。 He is healthier than me.
SheisthehealthiestpersonIknow。
Generally speaking, the comparative degree and superlative degree of adjectives have two forms: adding "-er" or "-est" before them, or adding "more" or "most". For some disyllabic or polysyllabic adjectives, such as healthy, the comparative degree can be in the form of "-er" or "more". However, in everyday English, people tend to use the form of "healthy" plus "-er" to express the comparative degree of "healthy".