The control group spent on average 36 days in the hospital, the group getting a single 500,000 IU dose of vitamin D3 spent an average of 18 days in the hospital.
This study isn't correlation, it was a double blind randomized control trial. However, it was a small pilot study and more research is needed.
The fact that this was based on only 31 people, showed a marginal effect (barely passed even the lenient p < 0.05 threshold) and doesn't seem to have been replicated in a bigger study despite being published 4 years ago should make you very sceptical about it. Sure, more research would be good, but nobody should be making any decisions based on this.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4939707/
The control group spent on average 36 days in the hospital, the group getting a single 500,000 IU dose of vitamin D3 spent an average of 18 days in the hospital.
This study isn't correlation, it was a double blind randomized control trial. However, it was a small pilot study and more research is needed.