Spades!! said "I've read its all about something called the 'viral load'.
Those people who are not vaccinated and get Covid will have the virus grow unchecked in their body, resulting in a high virus load. This person will easily spread a large amount of the virus and possibly get seriously sick from it.
A person who is vaccinated can still pick up the virus but not get as sick. The key in being vaccinated means the virus growth is limited because the vaccination has shown your immune system what this virus is. Your immune system can then act on the virus more effectively. This results in a lower virus load and you are less likely to spread it.
So whether you are vaccinated or not, if you get sick it is best to stay at home, try to not spread the virus. Wearing masks is a proactive measure that prevents respiratory spread of the virus in people who may not know they are infected yet. Hand sanitizing kills not only virus but bacteria that spread other illnesses.
The problem with the Delta variant is it is more infectious. It has been called the Pandemic of the Unvaccinated. This is the 4th wave coming. Most likely those people who remain unvaccinated will get hit hard.
Also because the vaccinations to Covid are new, we do not know how long they last in our body. We may or may not need a booster shot to keep Covid in check and possibly annual shots like the flu shot. Only time will tell.
Eventually we will figure out how to live with Covid. "
To add what you said (some of which I only learned this morning).
The functionality of the vaccine seems to prevent the viral load from amassing in the lungs and cardio-vascular system, where it does real damage. If you get sick even after infection, the viral load tends to stay in the upper respiratory system (with a similar viral load to non-vaccinated in that region), but doesn't seem to spread systemically as easily.
So a non-vaccinated person who gets ill will likely have the infection more through their body, increasing their overall viral load and impacted critical organ systems vs a vaccinated person. That's also why they're far more likely to be critically ill vs. a vaccinated person (something like only 1 in 70 critical ill patients will be vaccinated, with the other 69 being unvaccinated - that despite 50-75% of the adult population being fully vaccinated, depending on where you live).
It's that high viral load, coupled with the extended time that it takes an unvaccinated person to get better (due to viral load & virus novelity), that they think generates the variants. The virus is constantly producing variants, but 99.9+% of them amount to nothing or confers no advantage. It's that fraction of a fraction of a percent that DO confer advantage is how we end up with beasts like Delta.
Statistically, unvaccinated people have higher viral loads, meaning more replication is occurring, which increases the mathematical likelihood that a new variant of concern will be produced. Law of large numbers and all that.