Even bigger issue than base rate is that this is a retrospective study, so I suspect heavy measurement bias. Patients that are likely to have a Vitamin D level checked are different than hospitalized patients. Vitamin D levels are not checked routinely in hospitalized patients. I know this because I'm a hospitalist, but also because during the height of the local pandemic this 900+ bed hospital only had 216 patients with a vitamin D level checked. Presumably at least 1000 COVID patients moved through that hospital during this time, more likely several thousand.