Have you ever wondered why there is a “flu season”? I most often hear that it is because we are indoors more due to bad weather and in closer proximity to one another making transmission of infection easier. The problem I have had with that idea is that it is not mere exposure that makes us sick but the condition of our immune system at the time of exposure. That means that if we are healthy, exposure just helps to build antibodies like a vaccine.
I heard another idea for that phenomenon, in the Feb 25, 2008 Journal of Virology (Study of viruses), an article on flu stated that vitamin D improves our immune function and that during the winter decreased sun exposure leads to deficiency and thus increased susceptibility to infection. This one makes more sense to me so I started looking for other studies
A study in the 2-8-19 Journal Frontiers of Immunology found that the flu vaccine worked better in elderly persons given a vitamin D supplement.
6-10-18 Journal Nutrients looked at children with adequate levels of vitamin D to see if higher dosages made a difference, and did not see a difference. This was done in a couple of studies and they had similar conclusions like this one, “In conclusion, in healthy young children with sufficient vitamin D status, increasing vitamin D intakes does not confer additional advantage to immune function.” That wording leaves you with a less than enthusiastic feeling about the role of vitamin D in immune function. The problem is they are comparing two groups with adequate levels of vitamin D, but what if you are deficient?
According to the June 2018 Journal Cureus approximately 40% of the population is deficient in vitamin D. What if we compare a group of people getting vitamin D to placebo (which means they are probably deficient)? I found that in a study done May, 2010 in the American Journal of Clinical Nutrition 10.8% of the vitamin D group got sick versus 18.6% of the placebo group which means you are 58% less likely to get sick with vitamin D supplementation during winter.
According to the CDC, seasonal flu vaccine effectiveness for the years 2014-2019 was 19,48,40, 38,29. That means for the 2018-19 flu season the vaccine was 29% effective. The best year for effectiveness since 2004 was 2010-11 when it was 60%.
The take away here is you need vitamin D supplementation especially during the winter months when it is more difficult to get sun exposure. It has been more effective at preventing flu than the shot every year but one from 2004 to 2019.