Scenario A. 100 take a placebo and 100 get sick. 100 take the vaccine and only 10 get sick

Question

Therefore vaccine is 90% effective.

Scenario B. 20000 take a placebo and 100 get sick. 20000 take the vaccine and only 10 get sick.
What is the effectivity of the vaccine?

There must be a statistical difference between these two scenarios. And in both, the time period for each subject must be the same ( ie. waiting time from placebo ingestion and waiting time from vaccine injection must be the same ).