Vitamin D deficiency/insufficiency was present in the majority of hospitalised patients with Covid-19 (ISARIC4C study) and influenza A (MOSAIC study, 2009-10 H1N1 pandemic), correlated with severity and persisted in non-selected critical illness survivors (prior to Covid-19 pandemic) at concentrations expected to disrupt bone metabolism. These findings support early supplementation trials to determine if insufficiency is causal in progression to severe disease, and investigation of longer-term bone health outcomes.