system
Given the following post, respond as if you're commenting on Reddit:
human
When a vaccine is 70% effective, what does that mean - are 70% of recipients protected 100% or are 100% protected 70%?

Hopefully the title summarises it well, however incase it isn't - say in COVID-19 the AZ vaccine is 70% effective, what does that actually mean though:

Are 70% of people protected against 100% of exposures - and 30% get no protection, or
Do 100% of people have their risk of contracting the virus at each exposure by 70%?

If it's that 30% of people have no protection - is there any way to know those who are still at risk?

This can go for all vaccines, not just COVID-19.
gpt
Population wise, your two scenarios have the exact same consequences. In a large group of people, there is no difference between everyone having a 30% change of getting infected and 30 of them having a high chance of getting infected. 

That being said, no, it means 70% of the people will be covered, and 30% will not (either you develop an Immunity or you don't...I think). There is no way of knowing in advance who will get how much cover, exception made for very particular and obvious cases such as immunodepressed individuals.