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This depends on the r0 of the virus, an r0 of 2 means a 50% effective vaccine (and 100% vaccinated pop) will defeat the virus. With measles and a r0 of 18, youd need an effectiveness of 95%. Considering not everyone can or will take a vaccine, the effectiveness needs to be higher than this.

Covid is apparently around 3, and if you can vaccinate 90% of the population then an effectiveness of 75% would be the minimum you could get away with.

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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)

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