Just eat healthy and you won't get sick. I've never been vaccinated and I never get sick. I am also not overweight and I only eat fruits and veggies and some meat in small portions, and some nuts occasionally. The key is to not eat that much. And only eat nutrient dense foods. The body can fight off a lot of stuff if you are in healthy condition. If you are healthy, you prevent the problem from even happening. Of course, if you are old then you are at a great disadvantage because your body cannot recover as fast or be as baseline healthy.