An estimated one billion people around the world lack vitamin D, which increases their risk of COVID-19 infections and new strains of the virus that become complicated and potentially life-threatening. In November 2020, the authorities of Andalusia, a Spanish province, started giving vulnerable groups of people supplements of a particularly active form of vitamin D. The result of this intervention showed quickly. The number of patients in intensive care plummeted and the death rate dropped by 82 percent. Meanwhile, the death rate in Great Britain and many other countries went up, most likely because vitamin D deficiencies are more common during the winter period. A British politician has therefore urged Prime Minister Boris Johnson to follow the Andalusian method, which is both effective and inexpensive. Furthermore, a Spanish study shows that supplementing hospitalized COVID-19 patients with active vitamin D can save lives. How much vitamin D do we need and how does the body activate vitamin D from sun exposure or from supplements?