I’m not an epidemiologist, but I have spent a lot of time wrapping my head around epidemics. The reason 90% is “miraculous” is that it highly effectively reduces viral circulation.
Viruses are able to infect large groups of people only if transmissibility is easy.
Think of it like this. If an infectious person is likely to infect four other people while he is infectious, then we have a situation where infection rates surge on a steep curve. The progression is geometric meaning that it might start slow but will end up exploding.
But if an infectious person is likely to infect only one other person, then we have a linear situation in which infection rates remain stable and we have no explosive curve.
Now take a situation in which an infectious person is likely to infect fewer than one person. In that case, we’re going to see the infection rate drop and the total number of infections drop as well.
This is all called R value, by the way. If the effective R value for a population is greater than 1, infection rates inevitably increase. If the R value is less than one, an infection rates decrease.
So now let’s look at a vaccine. Say 80% of the US population achieves 90% immunity. This means four out of five people are highly unlikely to become infected even if exposed to the virus.
R value plummets and the virus is unable to propagate in the population after a period of time. The epidemic dies out.
So, even while a vaccine may not give you absolute assurance that as an individual you will never get covid19, it does assure from a population perspective that the virus will eventually cease to be a serious threat. Because if it’s not circulating, you can’t get it anyway.