There's a herd immunity threshold and it isn't 100%. No reason to create any requirement, as past that threshold it largely burns out.
/thread
Actually herd immunity doesn't mean that the spread stops, but that it drops down to a base level. It's like taking away 90% of the flammable material, but the remaining 10% can still burn.
Yes, theoretically, that is exactly what it means. The goal is to drop R0 (R nought) under 1, which means each person, on average, infects less than one person, let's say 0.5.
With an R0 of 0.5 100 people infect 50. Those people infect 25. And so on.
The purpose of the vaccine is to give the virus less targets and drop this R0 value under 1, so that subsequent generations (n) of the disease have increasingly lower numbers. There is no 'base level'. That would be an R0 of 1, where each person spreads it to 1 more, keeping number of infections at a stable number.
This is math. Has been from the start and still is now.
Realistically we are not talking about an isolated population in considering the plan for the US. This is likely endemic, much like the flu. and comes back seasonally, or cyclically, as it will circle the globe keeping itself alive, so this isn't a matter of vaccination of 100% of Americans, as we'll reach herd immunity here fairly quickly at this point between those already exposed and those vaccinated. The issue will be that it continues to circulate and evolve in other, poorer, countries and then swings back through. The challenge will be the seasonal inoculation of enough people to stop another outbreak and/or rapid identification of variants that are resistant.
If we had any sense we'd be giving people antibody tests to see if they had the antibodies and then vaccinating those that didn't first That would be the fastest path to herd immunity at this point to burn this down as rapidly as possible, as the problem has been shortage of the vaccine as companies ramp up to meet this massive global demand, but that's not what we're doing, opting instead to just mass distribute, which will get the job done eventually, but at the expense of vaccinating millions that don't actually need it while not vaccinating millions that are still out there spreading it around until we can pump out enough vaccine to just cover everyone. Panic has ruled since March and it still very much is on many levels.