Vitamin D helps reduce the proinflammatory cytokine and increases the anti-inflammatory cytokine.
There have also been countless studies about the affects of vitamin D deficiencies on mental and physical health:
Vitamin D and Depression: Where is all the Sunshine?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2908269/
Vitamin D Deficiency
https://medlineplus.gov/vitaminddeficiency.html
Vitamin D Deficiency, Its Role in Health and Disease, and Current Supplementation Recommendations
https://jaoa.org/article.aspx?articleid=2625276
With the lockdowns going on, people are getting less sunlight than previously, which can result in vitamin D deficiencies. According a previously mentioned study, over 80% of the covid patients had a vitamin D deficiency.
People are also spending drastically longer inside their homes than previously, not only getting less vitamin D from the sunlight, but also exposing themselves to an environment that might not be as "healthy" as you might think. There have been studies on the affects of staying indoors, and the air quality issues that might present an issue.
Indoor Air Can Cause Health Problems
https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=1&ContentID=2163
Why Being Cooped Up Indoors Is Seriously Bad for Your Health
https://www.phillymag.com/be-well-philly/2012/11/30/cooped-indoors-seriously-bad-health/
Indoor Staying During Winter Season Makes People More Susceptible to Flu
https://pubmed.ncbi.nlm.nih.gov/27426715/
Staying indoors can lead to respiratory diseases: 5 ways to improve air quality at work & home
https://economictimes.indiatimes.com/magazines/panache/staying-indoors-can-lead-to-respiratory-diseases-5-ways-to-improve-air-quality-at-work-home/articleshow/66747377.cms?from=mdr