"An effective immune response requires an adequate host nutritional status(Reference Calder7). In recognising this, the European Food Safety Authority has authorised nutrient function health claims for vitamins A (including β-carotene), B6, B9 (folate), B12, C and D, and the minerals Zn, Se, Fe and Cu based on scientific assessment of their contributions to the normal functioning of the immune system(8). Each of these micronutrients, as well as vitamin E, has been shown to have multiple key roles in supporting the immune system and reducing the risk of infections."
https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/optimising-covid19-vaccine-efficacy-by-ensuring-nutritional-adequacy/5F25F117DED5141638554BAAFC66E1FF
Optimising COVID-19 vaccine efficacy by ensuring nutritional adequacy
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7884658/