Clinical Trial to Optimise Levels of Vitamin D for Rhinovirus Protection

Inclusion Criteria: 1. Age 18-70 years 2. Gives written informed consent 3. Serum 25-hydroxyvitamin D concentration \<75 nmol/L 4. Agrees not to take supplement containing vitamin D during participation 5. Agrees not to commence smoking or vaping during participation Exclusion criteria: 1. Current smoker or vaper 2. Taken vitamin D supplement or other supplement containing vitamin D, in previous 3 months 3. Sunny holiday abroadI, defined as any location 51 degrees North/South of the equator, for ≥1 week, in the previous 3 months 4. Dependent(s) \<6 months old 5. Positive serology for anti-RV16 antibodies 6. Living with someone with severe airways disease 7. Any of the following medical conditions: 1. Diabetes mellitus 2. Asthma 3. Chronic Obstructive Pulmonary Disease 4. Respiratory allergies 5. Sarcoidosis 6. Hyperparathyroidism 7. Nephrolithiasis 8. Active tuberculosis 9. Liver failure 10. Renal failure 11. Lymphoma or other malignancy not in remission for ≥ 3 years