In an overview of the current state of research, a British-Dutch research group states:

• Treatment with micronutrients by medical professionals is safe and effective. It is a low-cost strategy to support optimal immune system function
• Therapy with some vitamins (such as vitamin C and D) above the official dose recommendations and below the safety limit is safe and effective.

Nutritients. 2020 Apr 23;12(4):1181. doi: 10.3390/nu12041181.

Optimal Nutritional Status for a Well-Functioning Immune System Is an Important Factor to Protect against Viral Infections

Philip C Calder  1 , Anitra C Carr  2 , Adrian F Gombart  3 , Manfred Eggersdorfer  4

You can read the entire article here.