It's a simple observation.
You are largely protected from death and severe illness after full vaccination as compared to the unvaccinated population. It's not difficult.
Just like you're largely protected from heart disease if you eat 10-15x the fiber than the average American. It's a cause and effect relationship with total cholesterol. Simple observation.
People who get the flu vaccine are actually healthier people than those who don't. That's because healthier people tend to make that decision. So, when you compare outcomes, you can't really do it unless you specifically hold bmi and lifestyle and race constant.
You can have a healthy thin group of vaccinated people that end up better off than the fatty, lazy unvaccinated who have problems. But the difference isn't the vaccine.
That's why you have to do actual science and keep other things constant. Because science.
And they don't even try it.