The underlying question is how much of the population has to be immune for "herd immunity" to take effect. The pandemic will burn out once enough people (let's

The underlying question is how much of the population has to be immune for "herd immunity" to take effect. The pandemic will burn out once enough people (let's say X% of the population) are immune to the virus, and this is called "herd immunity". Let's take the UK as our population. If a vaccine is 60% effective and we administer it to everyone in the UK, then about 60% of the UK's population will gain immunity to COVID. Another Y% of the population will have immunity to COVID already, even if the vaccine didn't work for them. Then the question is, is Y + 60 > X, thereby achieving herd immunity? You can see that if a vaccine is 95% effective, it's more likely to be above the herd immunity threshold of X%. Current estimates for X are about 70%.