Can be manufactured at a Chemical Plant (Tier 2) and distributed at a Hospital to allow faster recovery from alien plague infections.
Plague vaccines are useful in case any plague spores manage to infiltrate your colony. The vaccines are incredibly efficient - all the areas with hospital coverage will become pretty much immune to the plague for as long as the vaccines are in stock.
Vaccines will not be used if there are no infected colonists. As such, keep a stock of a couple hundred units on hand, otherwise you'll be pointlessly clogging up your storage depots.
Vaccine usage depends on number of colonists in the colony, not the number of actually infected colonists.