L-Arginine (1.6g) and Vitamin C (500mg) for 30 days in a big study (>1300) people shows big improvements compared to a multivitamin on a variety of subjective symptoms in Long Covid patients. These are not small improvements they are massive improvements in QoL for a lot of people.

This is the paper, well worth a read its not too long or overly complex. www.ncbi.nlm.nih.gov/pmc/articles...

Combining L-Arginine with vitamin C improves long-COVID symptoms: The LINCOLN Survey

Recent evidence suggests that oxidative stress and endothelial dysfunction play critical roles in the pathophysiology of COVID-19 and Long-COVID. We hypothesized that a supplementation combining -Argi...