Vitamin D, vitamin C and zinc are the three micronutrients that can be effective and safe to take in high dosages to strengthen your immune system against SARS-CoV-2 etc. Other vitamins and minerals also contribute to a strong immune system. Therefore, I also recommend a good multivitamin supplement as a solid foundation and to fill out any gaps.
Research indicates that especially the trace mineral selenium can be important in order to avoid serious illness from Covid-19. Vitamin A also plays a fundamental role for the immune system, but both vitamin A and selenium are nutrients that can result in side-effects in larger doses. Thus, it is best to only take them as a part of a multivitamin supplement, and to not take them as a single supplement in addition to that.
The impact of nutrition on the immune system is like a chain that is only as strong as its weakest link. For instance, if you are a vegan and therefore lack vitamin B12, then a B12 supplement may be the most important factor for both your immune system and overall health. On the other hand, if you are a man and a meat-eater, it can be a good idea to choose a multivitamin supplement without iron, as you could risk ingesting so much iron that it weakens the immune system.
Specifically, vitamin A in the form of retinol can lead to side effects in larger doses. You can take comparatively more of selenium before it becomes problematic. So, you can choose a multivitamin supplement that, for example, contains 100% of vitamin A and more than 100% of selenium, where the manufacturer refers to percentages of the official daily recommended intake.
For instance, I have a perfectly good product that contains 100% of vitamin A, 291% of selenium and no iron. This is well suited for a person like me. Half of the dosage, which corresponds to one pill instead of two pills a day, of the product in question would also be fine, whereby I can cut the financial cost in half. I eat quite a bit of carrots, which also contain vitamin A, but not in a form that can become toxic, even though I eat many carrots.
Among other things, the product also contains the following vitamins and minerals that have key functions in the immune system in different ways: vitamin B12, copper, manganese, iodine and molybdenum. The other vitamin B types, in addition to B12, are also important, and they are also included in a quality multivitamin supplement.
Additionally, some multivitamin supplements contain herbal blends that can increase the ability of the immune system to keep viruses down. You can ask your dietician or your health food store retailer what could be a good supplement for you.
Remember to take your multivitamin supplement with a meal that includes fat, so the fat-soluble vitamins can be absorbed.