Vitamin D: A Nutraceutical Supplement at the Crossroad Between Respiratory Infections and COVID-19

Even though in mid-2023 the World Health Organization declared the end of the public health emergency of international concern status for COVID-19, many areas of uncertainty about SARS-CoV-2 infection pathophysiology remain. Although in the last 4 years pharmaceutical industries widely invested in the development of effective antiviral treatments and vaccines, large disparities in their availability worldwide still exist, thus fostering the investigation of nutritional supplements as adjuvant therapeutic approaches for disease management, especially in resource-limited settings. During the COVID-19 pandemic, vitamin D has been widely used as an over-the-counter solution to improve disease evolution, thanks to its known immunomodulatory and anti-inflammatory actions. Ecological and observational studies support a relationship between hypovitaminosis D and COVID-19 negative outcomes and, according to this evidence, several research groups investigated the role of vitamin D supplementation in protecting from SARS-CoV-2 infection and/or improving disease evolution. This narrative review is intended to offer insights into the existing data on vitamin D's biological effects in respiratory infections, especially in COVID-19. Furthermore, it will also offer a brief overview of the complex interplay between vitamin D and vaccine-elicited immune response, with special attention to anti-COVID-19 vaccines.