There is this thing called the Susceptible-Infected-Recovered Model (SIR), which can be expressed as a system of partial differential equations, or carried out in discrete form. This can be parameterized using the reproduction number of a virus, and also the infectious period. Both of these inputs affect the rate of spread, and they are not independent variables. i.e., if we shorten an infectious period for a given virus, we will decrease the reproduction number. So, for instance, if you are infectious for 5 days instead of 10, you will infect less people, on average.
The whole point of a vaccine is to give your immune system practice in fighting off a virus. Kind of like a running workout. A virus can still invade your body, but it will be better prepared to fight it off, which means you will be less likely to die, and your infectious period will be shorter. This is not rocket science, and it is a known fact.
The whole point with COVID was to slow it down so we did not have 200 million infections in a short time period, because the reproduction number was high. There is a big difference between 2.9 (COVID) and 1.5 (common flu). SIR forecasts can be done on a simple spreadsheet. Measles has a reproduction number of 15! If a virus is unleashed on us with that high of a reproduction number, surely the hospital system would see massive failures.
That's all folks.