Vitamin D deficiency manifests itself as rickets (weakening of the bones) in children and osteomalacia (softening of the bones) in adults. Less severe vitamin D deficiency contributes to osteoporosis (bones are weaker and more likely to break). All of these are well documented outcomes in Western Europe.

More to the point, since Covid Researchers have posited that Vitamin D plays a critical role in preventing respiratory infections, reducing antibiotic use, and boosting the immune system response to infections. Vitamin D is important in regulation and suppression of the inflammatory cytokine response, which plays a role in the severe consequences of COVID-19 and ‘acute respiratory distress syndrome’ associated with ventilation and mortality in COVID-19. Vitamin D also helps the respiratory tract produce antimicrobial peptides that can lower viral replication rates.

RAPID EVIDENCE SUMMARY: VITAMIN D IN COVID-19 https://assets.gov.ie/static/documents/rapid-evidence-summar...

The causal role of vitamin d deficiency in worse Covid-19 outcomes: implications for policy and practice development https://repository.rcsi.com/articles/journal_contribution/Th...

Coronavirus disease 2019 and vitamin D https://pmc.ncbi.nlm.nih.gov/articles/PMC10277257/

Vitamin-D and COVID-19: do deficient risk a poorer outcome? https://www.thelancet.com/journals/landia/article/PIIS2213-8...