How about this: to be in the best possible position to beat any serious infection, you need to be in generally good health.
That means having adequate levels of all essential nutrients - i.e., all the vitamins, minerals, amino acids, enzymes etc that you get from maintaining a good diet and healthy lifestyle over the long term.
To focus on just one nutrient as a pandemic looms just isn't the answer; it's not how health works.
That said, I've personally researched what nutrients (note the plural) are most important for beating an infection like this, and have adjusted my nutrient intake somewhat, and recommended to close relatives they do the same. But this is after I and they have spent many years researching and maintaining good health practices.
Update: if I was in not-so-good health (and not in medical treatment for a diagnosed condition, in which case, consult your doctor), my approach would be:
- boost my amino acids, fatty acids and minerals with a few spoonfuls of hemp seeds every day
- take extra supplements of vitamins A, B complex, C and D (and perhaps K?)
- do plenty of walking in the fresh air and sun,
- keep a generally healthy diet of good quality protein and veg.
Update 2: Others are mentioning Vitamin K, which seems like a good idea to supplement if you're generally nutrient deficient, but may not be necessary if you have a good diet. [1]
Disclaimer: not a health professional, but have had personal reasons to get very knowledgable about health and diet over many years.
[1] https://www.healthline.com/nutrition/foods-high-in-vitamin-k