As a layman observer of the scientific debate, the Vitamin D guys have been quite persistent about the correlation between Vitamin D levels and deaths. Is there causality? Beats me. The best I've heard by way of explanation is that inflammation depletes Vitamin D stores. That idea is usually followed-up with the additional theory that CV-19 causes inflammation and high platelet activity, clogging up small blood vessels and producing most of the major effects that are observed.