There is no food or dietary supplement that enhances or strengthens the immune system in a healthy population without nutritional deficiencies. Although some vitamins (A, B6, B12, C, D) or minerals (zinc, selenium, copper, magnesium) have shown to play a role in the immune response to infections, none of them have shown to consistently prevent the risk of infection.
An exception would be the case of vitamin D which, in certain population groups, has shown to reduce respiratory infections. Recently, it has been suggested that the implementation of reference nutrient intakes of Vitamin D “might reduce the impact of COVID-19 in populations where Vitamin D deficiency is prevalent” (Martineua et al., 2020).
If you would like to know more about Vitamin D you can find more information in one of our articles: “A New and Highly Stable Cocrystal of Vitamin D3 for Use in Enhanced Food Supplements”