Metastudies strongly indicating effectiveness
“The results of the meta-analyses and respective TSAs suggest a definitive association between the protective role of vitamin D and ICU hospitalization.”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9864223/
https://pubmed.ncbi.nlm.nih.gov/34684596/
Study of prevention rather than post-treatment
“In a new study, researchers show a correlation between vitamin D deficiency and COVID-19 severity and mortality. The study is among the first to analyze vitamin D levels prior to infection, which facilitates a more accurate assessment than during hospitalization, when levels may be lower secondary to the viral illness.”
https://www.sciencedaily.com/releases/2022/02/220203161135.htm
HOW MUCH DO YOU NEED?
40-60ng/ml is the recommended blood serum goal to achieve, so the dose will be adapted to realise this goal. If deficient may need higher doses temporarily!
https://www.grassrootshealth.net/project/dcalculator/
Toxicity is very rare.
The definition of deficiency is set as too low by many countries according to the Endocrine Society. This results in misleading study results which claim there is no proven effect.
Many scientists and doctors are frustrated by the unscientifically low recommended doses by many health authorities and the lack of awareness campaigns
https://vitamindforall.org/letter.html
The Politics, Science and Media Involvement in the Vitamin D Pandemic. Background to Song “Vitamin D and Covid-19”
Definition of deficiency (note 1ng/ml = 2.5nmol/L)
https://eje.bioscientifica.com/view/journals/eje/180/4/EJE-18-0736.xml
Some other studies and recommendations
https://www.mdpi.com/2072-6643/13/10/3596
https://www.nature.com/articles/s41598-020-77093-z
https://www.bmj.com/content/371/bmj.m3872/rr-5
https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0239252#pone-0239252-g001
https://www.researchgate.net/publication/340409592_Evidence_that_Vitamin_D_Supplementation_Could_Reduce_Risk_of_Influenza_and_COVID-19_Infections_and_Deaths
https://medium.com/microbial-instincts/the-first-clinical-trial-to-support-vitamin-d-therapy-for-covid-19-906a9d907468
https://www.youtube.com/watch?v=0AySagDalY8
Statistics for the vitamin D “pandemic”
According to the Robertkochinstitut, up to 30.2% of adults in Germany have deficiency. 38.4% of adults achieve adequate care. 61.6% are therefore have suboptimal levels. This is the conservative estimate based upon lower definition of deficiency so it could be much more!
https://edoc.rki.de/handle/176904/2492Â (German)
“approximately 75% had suboptimal 25(OH)D concentration” (Asia)
https://www.mdpi.com/2072-6643/9/3/313
“Similar rates of vitamin D deficiency have been reported in Europe [26] and Canada. A greater prevalence of vitamin D deficiency exists in Middle Eastern countries. A study of 316 young adults aged 30-50 years from the Middle East showed that 72.8% had 25(OH)D values of less than 15 ng/dL (that is, severely deficient).”
https://www.medscape.com/answers/128762-54282/what-is-the-global-prevalence-of-vitamin-d-deficiency
“Moreover, more than two-thirds of all children had levels below 75 nmol/L, including 80 percent of Hispanic children and 92 percent of non-Hispanic black children. “ (USA)
https://medicalxpress.com/news/2009-10-latest-analysis-suboptimal-vitamin-d.html
“Suboptimal vitamin D status is a highly prevalent but treatable condition in both hospitalized patients and the general population “
https://pubmed.ncbi.nlm.nih.gov/18042130/
Vit D deficiency in India: 80-90%
https://www.youtube.com/watch?v=1Q_mhJRFtxg&t=79s
English 
German