by Dr. Mercola
Conventional health authorities claim getting a flu shot each year is the best way to ward off influenza. But where’s the actual science backing up that claim?
If you’ve repeatedly fallen for this annual propaganda campaign, you may be surprised to find the medical literature suggests vitamin D may actually be a FAR more effective strategy, and the evidence for this goes back at least a decade.
Dr. John Cannell, founder of the Vitamin D Council, was one of the first to introduce the idea that vitamin D deficiency may actually be an underlying CAUSE of influenza.
His hypothesis  was initially published in the journal Epidemiology and Infection in 2006.  It was subsequently followed up with another study published in the Virology Journal in 2008. 
The following year, the largest nationally representative study  of its kind to date discovered that people with the lowest vitamin D levels indeed reported having significantly more colds or cases of the flu. In conclusion, lead author Dr. Adit Ginde stated:
“The findings of our study support an important role for vitamin D in prevention of common respiratory infections, such as colds and the flu. Individuals with common lung diseases, such as asthma or emphysema, may be particularly susceptible to respiratory infections from vitamin D deficiency.”
Vitamin D Works Better Than Flu Vaccine If Your Levels Are Low
Since then, a number of studies have come to similar conclusions. Most recently, a scientific review [5,6] of 25 randomized controlled trials confirmed that vitamin D supplementation boosts immunity and cuts rates of cold and flu.
Overall, the studies included nearly 11,000 individuals from more than a dozen countries. As reported by Time Magazine: 
“… [P]eople who took daily or weekly vitamin D supplements were less likely to report acute respiratory infections, like influenza or the common cold, than those who did not …
For people with the most significant vitamin D deficiencies (blood levels below 10 [ng/mL]), taking a supplement cut their risk of respiratory infection in half.
People with higher vitamin D levels also saw a small reduction in risk: about 10 percent, which is about equal to the protective effect of the injectable flu vaccine, the researchers say.”
Like Cannell before them, the researchers believe vitamin D offers protection by increasing antimicrobial peptides in your lungs, and that “[t]his may be one reason why colds and flus are most common in the winter, when sunlight exposure (and therefore the body’s natural vitamin D production) is at its lowest …”
According to this international research team, vitamin D supplementation could prevent more than 3.25 million cases of cold and flu each year in the U.K. alone.  Another statistic showing vitamin D is a more effective strategy than flu vaccine is the “number needed to treat” (NNT).
Overall, one person would be spared from influenza for every 33 people taking a vitamin D supplement (NNT = 33), whereas 40 people have to receive the flu vaccine in order to prevent one case of the flu (NNT = 40).
Among those with severe vitamin D deficiency at baseline, the NNT was 4. In other words, if you’re vitamin D deficient to begin with, vitamin D supplementation is 10 times more effective than the flu vaccine.
Optimizing Vitamin D May Be Your Best Defense Against Influenza
In my view, optimizing your vitamin D levels is one of the absolute best flu-prevention and optimal health strategies available. Your diet also plays a significant role of course, as it lays the foundation for good immune function.
A high-sugar diet is a sure-fire way to diminish your body’s innate ability to fight off infections of all kinds by radically impairing the functioning of your immune system.
However, I do not agree that fortifying more processed foods with vitamin D is the best solution, although I realize it could potentially have a more widespread impact among people who remain unaware of the beneficial health effects of sunlight in general.
I believe sensible sun exposure is the ideal way to optimize your vitamin D. Taking a vitamin D3 supplement is only recommended in cases when you simply cannot obtain sufficient amounts of sensible sun exposure.
It’s also important to point out that, contrary to what’s reported by most mainstream media, including NPR report above, most people cannot optimize their vitamin D levels by getting the recommended 600 IUs of vitamin D from fortified foods. The dose you need really depends on your current blood level of vitamin D.
If it’s very low, you may need 8,000 to 10,000 IUs of vitamin D3 per day in order to reach and maintain a clinically relevant level of 45 to 60 nanograms per milliliter (ng/mL). The only way to know how much you need is to get tested at least once or twice each year.