If it was that easy corona would be a thing of a past. Instead, we'll be seeing a new wave in a month or so.
If majority of a nation took vaccine I don't think why would they care about the minority of those who didn't take. Hospitals wouldn't be filled thanks to vaccinated people and their risk is lower, as they claim to be.
Director from AZ already said how they don't know if 3rd dose of vaccine will be needed and time will tell. Some already give 3rd dose (Israel). It's all a testing field and there are no strict rules at this point.
I still think we'll be seeing old picture in a couple of months, despite of all those vaccines that have been rolled out.
First of all: there aren't many countries where (a big) majority has been fully vaccinated, so that's a bit of a moot point right now.
And yes, a 3rd shot or booster shot may very well be useful for "weaker" people, as their antibodies do seem to decrease significantly after 6 or so months.
As for caring about the minority, the point used to be how big that minority actually was. Whereas with the delta variant, a small minority looks like it could be enough for shit to hit the fan once more. So that's why people care. Even more so than 6 months ago, I guess.
Simply put:
Less people vaccinated => more virus circulation => most likely more people sick/in hospital/dead => probably more & more severe lockdowns
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Less people vaccinated => more virus circulation => higher chances of mutations => higher chance of a mutation for which current vaccines are less efficient => most likely more people sick/in hospital/dead (Talk about a self fulfilling prophecy for the antivax crowd!) => probably more & more severe lockdowns.
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Less people vaccinated => more virus circulation => the longer there will be all sorts of restrictions
&
...
Now, regardless of how you feel about taking a vaccine, the underlined bits are simple FACTS.