Purpose: The study null hypothesis is that vitamin D supplementation will not influence time to acute respiratory tract infection in sheltered accommodation residents.

The study population will comprise sheltered accommodation residents and staff fulfilling eligibility criteria. Half of the sheltered accommodation schemes will be randomly allocated to 2-monthly oral bolus vitamin D supplementation (Vigantol oil) over a period of one year, and half to placebo.

Participants will be followed for one year.

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