Trial of Vitamin D Supplementation for the Prevention of Influenza and Other Respiratory Infections (ViDiFlu)

Inclusion Criteria: * Permanent resident or member of staff at sheltered accommodation unit * If a woman of child-bearing potential, is sexually abstinent or has negative pregnancy test within 7 days of recruitment and agrees to use reliable form of contraception until she has completed the study * Able to give written informed consent to participate * Age ≥ 16 years on day of first dose of IMP Exclusion Criteria: * Current diagnosis of asthma or chronic obstructive pulmonary disease * Chronic upper or lower respiratory infection or other condition causing chronic cough * Condition requiring treatment with vitamin D at a dose of \> 10 micrograms vitamin D/day * Episode of upper or lower respiratory tract infection up to 28 days before first dose of IMP * Known sarcoidosis, hyperparathyroidism, nephrolithiasis, active tuberculosis, vitamin D intolerance, liver failure, renal failure, terminal illness, lymphoma or other malignancy not in remission at time of recruitment * Taking benzothiadiazine derivative, cardiac glycoside, carbamazepine, phenobarbital, phenytoin, primidone or long-term immunosuppressant therapy * Taking oral preparation containing \> 10 micrograms vitamin D/day up to 2 months before first dose of IMP * Using topical vitamin D analogue * Treatment with any investigational medical product or device up to 4 months before first dose of IMP * Breastfeeding, pregnant or planning a pregnancy * Baseline corrected serum calcium \> 2.65 mmol/L * Baseline serum creatinine \> 125 micromol/L * Inability to complete symptom diary with / without assistance * Cognitive impairment or communication problem precluding informed consent or successful completion of study evaluations