https://www.sciencedaily.com/releases/2015/04/150422121724.htm Respiratory infection is much higher among healthcare workers wearing cloth masks compared to medical masks, research shows.

Records from an "open-air" hospital in Boston, Massachusetts, suggest that (many) patients and staff were spared the worst of the outbreak of 1918. A combination of fresh air, sunlight, scrupulous standards of hygiene, and reusable face masks (for staff) appears to have substantially reduced deaths among some patients and infections among medical staff.

Studies have shown that ultraviolet radiation inactivates influenza virus and other viral pathogens and that sunlight also kills bacteria.

https://thevaccinereaction.org/tag/international-journal-of-molecular-sciences/

The groups who are at highest risk for severe COVID-19 match those who are at highest risk for severe vitamin D deficiency. This includes the elderly, men, ethnic groups whose skin is naturally rich in melanin (if living outside the tropics), those who avoid sun exposure for cultural and health reasons, those who live in institutions, the obese, and/or those who suffer with hypertension, cardiovascular disease, or diabetes.

https://pubmed.ncbi.nlm.nih.gov/33014983/