If 2025 taught us anything, it is that acute respiratory infections (ARIs) are no longer predictable or neatly seasonal. Australia experienced what public health experts described as a “multi‑wave” respiratory year, with an unexpected resurgence of Influenza A (H3N2) later in the season and sustained circulation beyond winter. Respiratory syncytial virus (RSV) notifications also reached record highs in the preceding year, placing continued pressure on primary care and reinforcing the need to rethink preventative strategies rather than relying solely on reactive care.¹

Against this backdrop, nutrients that support foundational immune competence deserve renewed attention, particularly vitamin D.

Despite Australia’s generous sunlight, vitamin D insufficiency remains common. Data from population studies suggest that a large proportion of adults sit below levels considered optimal for immune health, especially during winter and early spring when UVB exposure is reduced.² This matters because vitamin D plays an active role in immune defence as an important biological modulator of host responses to respiratory pathogens.³

A growing body of literature links low serum 25‑hydroxyvitamin D [25(OH)D] with increased susceptibility to respiratory infections. Meta‑analyses have demonstrated that vitamin D supplementation can reduce the risk of acute respiratory infections, with the greatest benefit seen in individuals who start out deficient.²

Importantly for Australian clinicians, this relationship has now been explored in a local context. In a 2025 study published in Advances in Integrative Medicine, Professor Karin Ried and colleagues assessed vitamin D status and respiratory illness frequency in Australian adults, followed by a targeted supplementation phase.²

The AIMED study* evaluated 199 adults and found that only one‑third had optimal vitamin D levels (>75 nmol/L) at baseline. Individuals with lower vitamin D status reported a higher frequency of colds and flus over the previous 12 months, with a statistically significant inverse relationship between serum vitamin D levels and respiratory infection frequency in those with insufficiency.²

Participants with sub‑optimal levels were then supplemented for three months using Vitamin D3^ at doses between 4,000 and 8,000 IU per day, tailored to baseline status. By the end of the intervention:

• 85% achieved optimal vitamin D levels.
• No participants remained deficient.
• Mean serum 25(OH)D concentrations increased into a range associated with immune protection.

The study reinforces two clinically relevant points: first, that sub‑optimal vitamin D status is common, and second, that well‑absorbed, practitioner‑designed vitamin D formulations can reliably and safely restore levels within a relatively short time frame.²

Beyond epidemiology, mechanistic research continues to clarify how vitamin D supports host defence against respiratory viruses. A recent comprehensive review highlights key pathways:³

• Enhanced innate immunity, including induction of antimicrobial peptides such as cathelicidin and defensins.
• Improved epithelial barrier integrity at mucosal surfaces.
• Balanced adaptive immune responses, supporting appropriate antiviral activity while moderating excessive inflammatory signalling.
• Cytokine regulation, reducing tissue‑damaging inflammation commonly associated with severe viral illness.

These effects are particularly relevant during acute infection and periods of heightened exposure, helping explain why adequate vitamin D status is associated with reduced infection severity and improved outcomes.

For Healthcare Practitioners, the take‑home message is practical: assess vitamin D status where appropriate and consider formulations with demonstrated bioavailability and clinical evidence behind them.

As 2025 reminded us, resilience starts with preparation and vitamin D remains a foundational tool in supporting immune readiness during an unpredictable respiratory landscape.

Footnote

*This study was led by Professor Karin Ried, a respected Australian researcher in nutritional and integrative medicine and Director of Research at the National Institute of Integrated Medicine (NIIM), whose work continues to strengthen the evidence base, guiding clinical use of vitamin D in immune health.

^ Vitamin D3 used in this study was provided by Metagenics.