[jbb67]

This is if everyone is equally likely to spread the virus.

Probably 20% of people are responsible for 80% of infections. (People who work in shops or public places, people in large extended families, people who socialize a lot).

If you can identify and vaccinate those people it will be much more that 20% effective. (It might be hard to identify those but it's probably easy to identify people at low risk of spreading it and infer)