evidence: Most of the previous hospital-based studies observed an association between vitamin D deficiency and a more critical outcome of COVID-19. Vitamin D deficiency has been associated with elevated risk of oxygen support43, ICU admission24, invasive mechanical ventilation11, severe sepsis/septic shock24, lung damage15, hypoxia13, death11,14,15, higher level of the inflammatory marker CRP13 and severe/critical COVID-19 illness defined according to severity scales developed by WHO or CDC13,14. However, other studies observed no association between vitamin D deficiency and clinical outcome of COVID-19, such as ICU treatment19, requirement for mechanical ventilation16,19, mortality16,19,25, length of hospitalization17,19 and severe/critical COVID-1917 (WHO definitions). [...] Pereira, M. et al. Vitamin D deficiency aggravates COVID-19: Systematic review and meta-analysis. Crit. Rev. Food Sci. Nutr.62, 1308–1316 (2022). [...] Jude, E. B. et al. Vitamin D deficiency is associated with higher hospitalisation risk from COVID-19: A retrospective case-control study. J. Clin. Endocrinol. Metab.106, e4708–e4715 (2021).