The impact of vitamin D deficiency on COVID-19 severity in the UK: a systematic literature review

Background: Vitamin D deficiency affects 20%–40% of the UK population, with higher prevalence among ethnic minorities, older adults, and individuals with obesity.1 Deficiency impairs immune responses, increasing susceptibility to respiratory infections such as SARS-CoV-2 [2]. Emerging evidence links low vitamin D levels to greater COVID-19 severity through dysregulated inflammation and impaired ACE2 modulation, although findings remain inconsistent.3 Objectives: To evaluate whether vitamin D deficiency independently predicts severe COVID-19 outcomes in UK populations and to examine evidence supporting vitamin D supplementation for prevention or adjunctive therapy. Methods: Following the PRISMA 2020 guidelines, studies were systematically identified through PubMed, Scopus, and Google Scholar searches covering the period from 2019 to 2025. A comprehensive search strategy was employed, combining terms such as (“vitamin D deficiency” OR “Vit D” OR “25(OH)D”) AND (“COVID-19 severity” OR “SARS-CoV-2”) AND (“UK” OR “United Kingdom”). Studies were independently screened, and quantifiable data on serum vitamin D levels, hospitalization, ICU admission, and mortality were extracted into Excel. Qualitative data regarding mechanisms and UK-specific risk factors were thematically synthesized. Descriptive statistics summarized quantitative findings. Study quality was appraised using the Mixed Methods Appraisal Tool. Ethical approval was not required. Results: Of the 2030 articles screened, 13 met the inclusion criteria. Two observational studies indicated that vitamin D deficiency (serum levels