booster dose vs dosage

booster dose and dosage both are nouns.

Word	Noun	Adjective	Verb	Adverb
booster dose	Yes	No	No	No
dosage	Yes	No	No	No

As nouns, dosage is a hypernym of booster dose; that is, dosage is a word with a broader meaning than booster dose:

booster dose: an additional dose that makes sure the first dose was effective
dosage: a measured portion of medicine taken at any one time

Other hypernyms of booster dose include dose.

booster dose (noun)	dosage (noun)
an additional dose that makes sure the first dose was effective	a measured portion of medicine taken at any one time
	the quantity of an active agent (substance or radiation) taken in or absorbed at any one time