Boosting is the wrong model
An overactive immune system causes allergy and autoimmunity. The goal is a well-supplied, well-regulated system, not a boosted one.
Nutrients that matter
Deficiency in vitamin D, zinc, vitamin C, vitamin A, selenium or protein measurably impairs immune function. Correcting a deficiency helps; taking more once you're replete generally doesn't.
The unglamorous basics
Sleep, physical activity, not smoking, stress management and adequate protein outperform any supplement stack for infection resistance and vaccine response.