Its interesting to see how engineers approach this kind of problem.
Please do not consider this thread as medical advice but sharing healthy ideas, tips, etc.
1. Vitamin C, at least 3x1g (or more, up to 3x4g, powder form)
2. Retynol, 10-25K IU - essential for respiratory and muccous diseses
3. Mg-Citrate, 2x200mg (activates C SVCT receptors) and good on its own
4. D3, at least 5000 IU or more (no more then 50k)
5. Fish oil, 3/4 tbsp (moleculary distiled, highly concentrated) - for more vitamin A, as CoX inhibitor and genral immune booster
6. Zinc, 10-50mg - in combination with C its known anti-flue agent
7. Some good probiotic - you know what fights viruses like a champ ?
8. CoQ10, K2 - for energy
There is no downsides to this, if you don't count money lost.
NOTE: I am not a doctor but software engineer who doesn't think outsourcing something as essential as your own health is reasonable, and hence learns medicine daily for more then decade. Do reserch on your own.