Epidemic influenza and vitamin D

In 2006, Dr. John Cannell and colleagues published a provocative hypothesis: the seasonality of influenza epidemics might be explained by the seasonal variation in vitamin D levels. This paper helped launch a new field of research into vitamin D and respiratory immunity.

The Puzzle of Influenza Seasonality

Why does influenza peak in winter? The traditional explanation—that cold weather keeps people indoors, facilitating transmission—has problems. Influenza spreads in tropical climates without cold seasons. And indoor crowding occurs year-round in schools and offices without triggering summer epidemics.

The Vitamin D Hypothesis

Cannell proposed that the real explanation involves vitamin D:

• •Winter sun exposure is insufficient for vitamin D synthesis at latitudes above 35°
• •Blood levels drop dramatically between fall and spring
• •Immune function becomes impaired as vitamin D falls

This would explain why flu epidemics follow the sun, not the thermometer.

Biological Plausibility

The authors detailed the mechanisms by which vitamin D could affect susceptibility to respiratory infections:

• •Cathelicidins: Vitamin D activates genes for antimicrobial peptides that kill bacteria and viruses
• •Toll-like receptors: Vitamin D enhances the recognition of pathogens by immune cells
• •Inflammatory response: Vitamin D modulates cytokine production, potentially reducing the dangerous "cytokine storm" in severe infections

Historical Evidence

The paper noted that cod liver oil—rich in vitamin D—was the standard treatment for tuberculosis before antibiotics. Sanatoriums for TB patients emphasized sunlight exposure. These practices may have worked partly through vitamin D.

Epidemiological Patterns

African Americans have lower vitamin D levels due to darker skin pigmentation and die from influenza at higher rates than white Americans. This correlation supports the hypothesis, though it does not prove causation.

This paper, though speculative, helped spark the research that would later confirm vitamin D's role in respiratory immunity through studies like Martineau 2017.