Questions about example sentences with, and the definition and usage of "Effectivity"

A: "Effectivity" sounds unnatural and I would not use it.

"They are testing the effectiveness of the new COVID vaccine."

A: Effectivity is an noun which signifies how effective something is, efficacy is the quality of being able to do something in an effective manner. Efficiency is how well something is done, but not in the same sense as something being effective. If something is efficient, it works well and usually in a timely manner. If something is effective, it works in general. Effectivity is more broad than efficiency. Effectiveness is pretty much the same as effectivity.