Efficacy is still tested with vaccines because there is a statistical expectation of contracting a given disease.
If you give 40,000 people a random mix of placebo and vaccines and then monitor whether or not they contract the target disease, you get an indication of efficacy by comparing the vaccinated group to the placebo group.
If you give 40,000 people a random mix of placebo and vaccines and then monitor whether or not they contract the target disease, you get an indication of efficacy by comparing the vaccinated group to the placebo group.