Abstract
Background: Vitamin D deficiency has been associated with a number of diseases, including influenza. Whether or not this reflects a causal relationship is unknown. We therefore wanted to examine if supplementation with vitamin D would affect the incidence and severity of influenza-like disease. Methods: Questionnaires on influenza were sent to subjects participating in ongoing placebo-controlled intervention studies with vitamin D supplementation, up until the end of April 2010. Results: Five hundred and sixty-nine subjects from 10 different clinical trials were included in the study, of whom 289 were randomized to receive vitamin D (1111–6800 IU/day) and 280 to receive placebo. Influenza-like disease during the previous fall/winter was reported in 38 subjects in the vitamin D group and 42 in the placebo group (non-significant), of whom 25 and 26 subjects, respectively, fulfilled our clinical criteria for influenza. In these latter subjects, the duration of illness was significantly longer among those in the vitamin D group than among those in the placebo group (median 7 (range 2–60) days vs median 4 (range 2–18) days; p = 0.007). However, this difference was not statistically significant if all 38 (vitamin D) and 42 (placebo) subjects who reported symptoms were included. Conclusion: Our results do not support the hypothesis that high doses of vitamin D supplementation will have a pronounced effect on influenza-like disease in populations not targeted for high influenza risk.
Acknowledgements
The superb assistance of the staff of the Clinical Research Unit, University Hospital of North Norway is gratefully acknowledged.
Declaration of interest: The present study received funding from The Northern Norway Regional Health Authority, The Norwegian Research Foundation, The Chief Scientist Office, Scottish Government, Chest Heart and Stroke Scotland, The National Institutes of Health grants 5KL2RR025015, P30 DK017047, and UL1 RR025014. None of the authors have any financial involvement or affiliation with any organization whose financial interests may be affected by material in the manuscript, or which may potentially bias it.