Vitamin D supplementation did not prevent influenza-like illness as diagnosed retrospectively by questionnaires in subjects participating in randomized clinical trials.

Scand J Infect Dis. 2011 Oct 25.
Jorde R rolf.jorde at unn.no , Witham M, Janssens W, Rolighed L, Borchhardt K, Boer IH, Grimnes G, Hutchinson MS.
Endocrinology Research Group, Institute of Clinical Medicine, University of Tromsø, and Medical Clinic, University Hospital of North Norway , Tromsø , Norway.

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.
PMID: 22026455
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Abstract does not indicate

1. Existing vitamin D levels. We assume that subjects were all in Norway – so probably already had high level of vitamin D due to supplementation, cod liver oil, or tanning beds
2. If the subjects were taking vitamin A (cod liver oil) which cancels much of the benefit of Vitamin D
3. If vitamin D was given before the flu season started.– Vitamin D can take months to re-stock and then help
4. Difference results for different vitamin D doses
   Answer by author: There was no significant difference between those taking high vs low dose.

See also Vitamin D Life

• Overview Influenza and vitamin D