There is no guarantee that a protective effect that helps against rhinovirus or influenza will cross over and help with coronavirus but it might and most of these supplements are safe with limited downsides.
The following are two supplements I have purchased and have on hand to take immediately if I develop any respiratory symptoms. I have also listed the studies that show these agents appear to be helpful at least with influenza. Hopefully their protective effect will cross over to coronavirus although this is unknown.
Elderberry Extract
Randomized study of the efficacy and safety of oral elderberry extract in the treatment of influenza A and B virus infections.
https://www.ncbi.nlm.nih.gov/pubmed/15080016
Elderberry Supplementation Reduces Cold Duration and Symptoms in Air-Travellers: A Randomized, Double-Blind Placebo-Controlled Clinical Trial
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4848651/
Vitamin C
The effectiveness of vitamin C in preventing and relieving the symptoms of virus-induced respiratory infections.
https://www.ncbi.nlm.nih.gov/pubmed/10543583
The relationship between vitamin C dosage and its effects on the duration of the common cold symptoms.
https://www.sciencedaily.com/releases/2017/03/170330115246.htm
What I am personally doing it taking 1,000mg of vitamin C daily and if I develop symptoms of respiratory illness I will start taking the elderberry.
Keto diet!
Strengthens mucous virus barriers.
Read:
https://www.ncbi.nlm.nih.gov/pubmed/31732517
https://www.inverse.com/article/60949-keto-diet-influenza-fight-infection
Don't trust pseudoscience. Taking vitamin C makes much more sense as a preventative measure.
Diarrhea guaranteed!
For the extra cleansing effectzx