"Patients with vitamin D deficiency (<20 ng/mL) were 14 times more likely to have severe or critical [Covid] disease than patients with 25(OH)D ≥40 ng/mL."
Peer-reviewed study: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0263069
And funnily enough one of the strongest adjuvants to Vitamin D is sunlight. Pretty hard to get your adequate fill of under lockdowns, watching Pedoflix and eating Uber Shits.