Vitamin D's Role in Fighting COVID-19, Long COVID, and Influenza | Research

The takeaway

Review summarizes evidence that vitamin D modulates immune responses to COVID-19, Long COVID, and influenza through antimicrobial peptide induction and immune regulation.

LL-37 antimicrobial peptide
Vitamin D induces this key antimicrobial peptide that directly kills respiratory pathogens

What the researchers found

Vitamin D modulates infectious disease outcomes through antimicrobial peptide induction (cathelicidin/LL-37), immune regulation, and anti-inflammatory effects in COVID-19, Long COVID, and influenza.

Why it matters

Vitamin D deficiency is extremely common worldwide and easily correctable. If it truly modulates respiratory infection outcomes, supplementation could be a simple, cheap public health intervention.

How the study worked

Narrative review of immunological, clinical, and preventive evidence for vitamin D in COVID-19, Long COVID, and influenza.

What this study cannot tell us

Narrative review — clinical trial evidence for vitamin D in respiratory infections remains inconsistent. Optimal dosing and timing are unclear.

How to read the evidence

Narrative review — synthesizes observational and trial evidence with mixed clinical results.

When this study was published

Published in 2026; covers post-pandemic vitamin D research.

The bigger picture

This connects vitamin D to the antimicrobial peptide defense system, showing how a common nutrient deficiency could impair innate immunity against respiratory infections.

Questions still open

What is the optimal vitamin D level for immune protection?
Does vitamin D supplementation help with Long COVID symptoms?

Common questions

Does vitamin D help fight COVID-19?

Vitamin D stimulates production of antimicrobial peptides and regulates immune responses. Observational data suggests adequate levels may reduce COVID-19 severity, but clinical trials have shown mixed results.

How does vitamin D boost immunity?

Vitamin D induces production of antimicrobial peptides like LL-37 that directly kill pathogens, while also regulating inflammatory responses to prevent immune overreaction during infections.

Read the original research

Vitamin D in Infectious Diseases: A Narrative Review Focusing on COVID-19, Long COVID, and Influenza.

Nutrients, 18(4)

View on PubMed ↗Publisher’s paper ↗

Citation

Caliman-Sturdza, Olga Adriana; Gheorghita, Roxana Elena; Soldanescu, Iuliana; Dimian, Mihai; Mangul, Serghei. (2026). Vitamin D in Infectious Diseases: A Narrative Review Focusing on COVID-19, Long COVID, and Influenza.. Nutrients, 18(4). https://doi.org/10.3390/nu18040634