> With 90% efficiency it will be extremely tough for the virus to spread

Aren't you assuming 100% coverage at 90% effectiveness?

What if it's - say - only 50% coverage, at 90% effectiveness. That's not looking so good.

I'm still trying to understand why an otherwise young and healthy person would want to get take one, or more likely multiple, shots of a C19 vaccine, given the risk from C19 is so low. What's the benefit to them?

In HN-speak: how well does a C19 vaccine deployment scale?