Efficiency is lack of waste. In some cases, it can mean "lack of waste of money" or "lack of waste of energy".
Example: You're selling widgets, and you want to make the most money possible. This is an efficiency goal. Widget production cost = waste + materials + labor.
Efficacy is effectiveness. It can mean "level of coverage" or "level of positive outcome".
Example: You're making a vaccine, and you want to make sure everyone gets it. Price is no longer particularly important, only the level of coverage. This is an efficacy goal.
Number of people dead = ((people in population) / (people covered))^2 (since transmission is reduced, it's square-law-type)