: Recent research from 2025 and 2026 suggests that low vitamin D levels may also contribute to the development of long COVID . Deficient patients were reportedly 5.8 times more likely to experience persistent symptoms such as fatigue and brain fog months after their initial recovery. Biological Mechanisms

Vitamin D is believed to modulate the immune system through several pathways: Vitamin D and COVID-19: Clinical Evidence and ... - PMC

: Patients with vitamin D deficiency were found to be up to 14 times more likely to have a severe or fatal disease course in some observational studies.

: Meta-analyses indicate that low vitamin D status increases the risk of ICU admission by more than twofold and the risk of death by 1.22- to 4.15-fold. In a large study, mortality among deficient patients was 25.6%, compared to just 2.3% for those with sufficient levels.

Low vitamin D levels are consistently tied to an increased risk of severe COVID-19, including higher rates of intensive care unit (ICU) admission and mortality. Multiple studies and meta-analyses suggest that deficient patients—those with serum levels typically below 20 ng/mL—are significantly more likely to experience severe respiratory symptoms than those with sufficient levels. Key Findings on Vitamin D and COVID-19 Severity