Speaker's Highlight

• Don Sin University of British Columbia, St. Paul Hospital (Canada)
  Kenneth R. Chapman Toronto General Hospital Research Institute (Canada)
• Parameswaran Nair McMaster University (Canada)
  Carolyn Calfee UCSF (U.S.A.)
• Gregory P. Downey University of Colorado School of Medicine (U.S.A.)
  David A. Schwartz University of Colorado School of Medicine (U.S.A.)
• Neil Schluger Tuberculosis Control Branch, California Department of Public Health (U.S.A.)
  Nick Kim Critical Care & Sleep Medicine, University of California San Diego (U.S.A.)
• Nicola Hananiah Baylor College of Medicine (U.S.A.)
  Jae-Joon Yim Seoul National University College of Medicine (Republic of Korea)
• Koichiro Asano Tokai University School of Medicine (Japan)
  Diahn-Warng Perng Taipei Veterans General Hospital (Taiwan)
• Konstantinos Kostikas University of Ioannina (Greece)
  Karin Klooster University Medical Center Groningen (Kingdom of the Netherlands)

• Vitamin D deficiency and respiratory infections: what the evidence shows

  Vitamin D has long been recognised for its role in musculoskeletal health, but its influence on immune function has become a growing area of clinical interest. Australian researchers have contributed substantially to this field, particularly in a country where respiratory illness remains one of the most common reasons for general practice consultations and hospital admissions.

  The relationship between low vitamin D status and respiratory tract infections has been examined in observational studies, mechanistic laboratory work, and randomised controlled trials. In Australia, where UV exposure varies dramatically between Darwin and Hobart, the epidemiology of deficiency is uniquely shaped by latitude, lifestyle and skin type, giving local findings a distinctive character.

  This overview synthesises the current evidence on how inadequate vitamin D levels influence the risk, severity and recurrence of respiratory infections, with practical context for clinicians, researchers and the wider Australian community.

  The role of vitamin D in immune defence

  Vitamin D acts on both innate and adaptive immunity through the vitamin D receptor, which is expressed in a wide range of immune cells. It stimulates the production of antimicrobial peptides such as cathelicidin, which line the respiratory epithelium and form a first chemical barrier against viruses and bacteria.

  When serum 25-hydroxyvitamin D falls below 50 nmol/L, the threshold commonly cited in Australian pathology reports, these immune functions are blunted. Laboratory work conducted at the Garvan Institute in Sydney and at other Australian research centres has demonstrated that monocytes and macrophages exposed to adequate vitamin D mount a stronger response to respiratory pathogens.

  This immunological backdrop offers a plausible biological mechanism for why inadequate levels have been repeatedly associated with infections ranging from the common cold to community-acquired pneumonia.

  Why Australians face a surprising deficiency risk

  Despite the country's reputation for abundant sunshine, deficiency rates remain substantial. The Australian Bureau of Statistics' national health measures survey found that roughly one in four adults had a serum 25-hydroxyvitamin D level below 50 nmol/L, with prevalence climbing to more than one in three during winter months.

  Latitude is a significant driver. In Hobart, Melbourne and parts of Adelaide, the UV index drops too low for cutaneous vitamin D synthesis for several months of the year. Conversely, in Brisbane, Perth and Darwin, year-round UV is strong enough that deficiency usually reflects lifestyle factors, including indoor work, cultural dress, and the strict sun-avoidance behaviours promoted by Cancer Council Australia's long-running Slip-Slop-Slap campaigns.

  Other at-risk groups include people with darker skin, older residents of aged care facilities, housebound patients and those with malabsorption conditions. Data from the Royal Australian College of General Practitioners indicates that testing rates have increased steadily over the past decade, particularly in the southern states during the colder half of the year.

  Common respiratory infections linked to low vitamin D

  The infections most consistently associated with deficiency are acute viral illnesses such as influenza and respiratory syncytial virus, along with bacterial pneumonia and tuberculosis. Vitamin D's influence on cathelicidin production and on the regulation of inflammatory cascades helps explain this breadth of association.

  In Australian paediatric cohorts, children admitted to hospital with bronchiolitis have been shown to have lower vitamin D levels than matched controls, a finding consistent with international data. Similar patterns have been reported in Aboriginal and Torres Strait Islander communities, where hospitalisation rates for lower respiratory tract infections remain disproportionately high.

  Chronic respiratory disease also intersects with vitamin D status. Studies based at the Royal Prince Alfred Hospital and other tertiary centres have reported that deficient patients with chronic obstructive pulmonary disease experience more frequent exacerbations, often triggered by bacterial or viral infection.

  Research findings from clinical studies

  A landmark meta-analysis pooling data from 25 randomised controlled trials, including several conducted in Australian teaching hospitals, concluded that supplementation reduced the risk of acute respiratory infection, with the strongest benefit observed in participants who began with severe deficiency.

  The protective effect appeared greater with daily or weekly dosing than with large monthly boluses, a finding that has shaped Therapeutic Goods Administration guidance on product labelling. Australian prescribers now generally favour modest daily doses over intermittent high-dose injections, which some earlier trials had linked to a transient rise in adverse events.

  Not every study has shown benefit. Trials enrolling participants with already adequate baseline vitamin D, or those using single annual doses, have produced neutral results. The interpretation gaining traction is that supplementation is most useful when it corrects a genuine deficiency rather than boosting already sufficient levels.

  Testing and supplementation in Australian practice

  Pathology testing for 25-hydroxyvitamin D is widely accessible through Medicare, although specific MBS item numbers apply and frequent repeat testing is discouraged unless clinically indicated. Most Australian laboratories report results in nmol/L, with the Royal College of Pathologists of Australasia recommending a target range of 50 to 75 nmol/L for the general population.

  For deficient patients, treatment typically begins with a loading regimen of 3000 to 5000 IU daily for several weeks, followed by maintenance of 1000 to 2000 IU daily. Over-the-counter preparations sold in Australian pharmacies vary in formulation, and the TGA has flagged inconsistent dosing accuracy in some imported products, prompting renewed interest in locally manufactured supplements.

  Lifestyle advice remains important. Brief, regular sun exposure outside peak UV hours, dietary sources such as oily fish, eggs and fortified milk, and weight-bearing exercise all complement supplementation. For patients in aged care or with limited mobility, clinicians may need to coordinate with dietitians and physiotherapists to maintain adequate intake and overall bone health.

  Public health considerations and prevention strategies

  At a population level, the policy challenge is balancing sun safety with vitamin D sufficiency. Australia's sun protection campaigns are among the most successful in the world and have driven down melanoma rates, yet they may inadvertently contribute to deficiency in certain groups when applied too rigidly throughout the year.

  Targeted approaches tend to work better than blanket advice. The Australian Institute of Health and Welfare has highlighted the need for tailored messaging for at-risk communities, including recent arrivals from countries where women traditionally cover their skin, older Australians living alone, and people with disability receiving home-based care through the National Disability Insurance Scheme.

  For clinicians attending respiratory medicine gatherings such as APSR 2022, the practical implication is straightforward. A brief assessment of vitamin D status, especially in patients with recurrent or severe respiratory infections, can identify a modifiable risk factor that responds to inexpensive, widely available treatment supported by consistent national guidance.

  The evidence linking inadequate vitamin D to respiratory infections is neither incidental nor absolute. It points to a modifiable contributor to immune resilience, particularly relevant in a country with diverse climates and a population whose deficiency risk is shaped as much by behaviour, age and skin type as by latitude. The key message for practitioners is to assess, test where appropriate, and treat to within the recommended range, while continuing to reinforce balanced sun exposure and dietary sources as the foundation of long-term respiratory health.

  Richard Russell Nuffield Department of Clinical Medicine, University of Oxford (United Kingdom)
• Mona Bafadhel King’s College London (United Kingdom)
  David Jackson Guy’s and St Thomas’ Hospital, King’s College London (United Kingdom)
• James Chalmers University of Dundee (United Kingdom)
  David Price University of Aberdeen (United Kingdom)