During the winter season, the question of acquiring the flu during colder weather becomes a concern. It is estimated that 5 to 20% of various populations contracts influenza each year. The truth is that the lack of vitamin D producing rays in winter time, at least in the northern hemisphere, is due to the lack of sunlight which increases our susceptibility to the flu. 
Evidence abounds that vitamin D plays a major role in regulating the immune system and a deficiency has been shown to increase the risk of influenza and respiratory tract infections. While the mechanism of vitamin D has not been fully elucidated, its protection against viral illnesses and respiratory tract infections are believed to be related to its effect on anti-microbial peptides and human beta defensin 2. , 
To help stave off the flu, ensure that you’re taking enough vitamin D3 to bring your 25-hydroxy-vitamin-D level between 60 to 90 ng/ml. To reach this range, most people will need to supplement with 10,000 IU daily or possibly more, depending on skin color, distance from the equator, and exposure to the sun’s rays. Consult with your doctor about vitamin D testing.
To perform an initial vitamin D assessment at home, order a test kit from Vitamin D Council.
In addition to vitamin D, please be aware that supplementing or ensuring adequate intake of vitamin A is necessary for optimal immune balance. Not everyone has the capacity to convert plant based vitamin A into all trans retinoic acid (ATA), therefore, considering supplementing with either retinal palmitate or an animal based form. I suggest 10,000 International units 3 to 7 days per week. More information about vitamin A, involving short-term higher dosages later on in this article.
Reversing the Flu
Despite perfect vitamin D levels, it’s still possible to succumb to influenza anyway. If this happens, there are some natural ways to reduce flu symptoms and get you back on your feet in a hurry. However, let’s first address the obvious things that can sometimes be overlooked.
Nothing will work to prevent illness unless one has adequate sleep while keeping stress under control. Lack of sleep, stress or both is a combination doomed to fail
any success in preventative measures. The immune system takes a serious hit if it’s overwhelmed with either affliction.
Consider the use of vitamin C for mild to severe complications from the flu, including pneumonia. Vitamin C is a potent anti-viral and anti-bacterial nutrient when delivered in pharmacological doses, especially intravenously (IV). ,  Significantly boosting levels of vitamin C will also raise the body’s glutathione levels, which will help remove toxins.
During the period of 1943 through 1947, Dr. Frederick Robert Klenner reported successful treatment of 41 cases of viral pneumonia, often using high-dose, IV driven vitamin C. , 
Dr. Robert Cathcart, who has specialized in very high therapeutic doses of vitamin C, recommends an IV dose for most types of the flu from 100,000 to 150,000 mg/day. 
Since IV delivery isn’t the most attractive option, it’s lucky for us that a relatively new oral form called Lypo-Spheric vitamin C has been shown to be equally effective. This delivery method offers advantages over standard forms of oral vitamin C, yet is nearly or equally as effective as moderate IV doses.
Lypo-Spheric vitamin C is a liposomal-encapsulated form that utilizes an essential phospholipid, called phosphatidylcholine, which forms extremely small bubbles or spheres, approximately 300 nanometers in diameter. The vitamin C is then encapsulated within these spheres, allowing it to maintain its integrity through the digestive tract. It is then absorbed into the blood stream and the vitamin C is delivered directly into cells with full effect. 
Depending upon severity of the flu, take one or more packets of Lipo-Spheric Vitamin C twice daily until fully recovered. For very severe cases of the flu, contact your doctor for intravenous vitamin C and monitoring.
Another effective treatment for flu symptoms is a short course of high-dose vitamin A in the form of retinol or retinyl palmitate (not beta carotene), which can kick start the immune system to help fight off the flu.  The dosage for short-term use of vitamin A is between 25,000 and 100,000 IU per day for a period of ten days.
Vitamin A has also been shown to work synergistically with vitamin D. In simple language, vitamin D produces a natural anti-biotic, while vitamin A cleans up the mess left by what vitamin D has destroyed. The two together make a marvelous tandem and help keep the immune system in proper balance.
What About the Flu Shot?
The American Medical Association (AMA) and the Centers for Disease Control (CDC) firmly believe that influenza vaccinations are “necessary” for the public health, and the media is diligent in reminding us every autumn that it’s time to get our shots, especially children and the elderly.
But according to a review of 51 studies involving over 290,000 children between 6 to 24 months of age, receiving a flu shot was not any more effective than either having no shot or receiving placebo.  An interesting note for parents: one of the studies did show flu shot efficacy 33% of the time in children over the age of two. Proper vitamin D levels will provide adequate protection without the side effects that may accompany a flu shot, but this study did show some positive results.
Perhaps the most compelling evidence against flu shots comes from Japan. In 1976, they started mandatory bi-annual vaccinations of 17 million school children, but as evidence began to mount about the questionable efficacy of flu shots, a study was conducted in 1984 comparing flu incidences to vaccinations in different cities. The results:
As you can see, flu shots had little effect on the overall incidence of influenza and, as a result, Japan ended mandatory vaccinations in 1987. 
Flu Shots Lack Evidence Of Safety
Another problem with vaccinations is that manufacturers have to guess which influenza strains will be most active 9 months before flu season starts so that they can produce and distribute shots. If the virus mutates or companies predict the wrong strains, the resulting flu shots won’t provide the level of protection people expect.
Finally, natural immunity is significantly more powerful than provoked immunity. Vaccination invites unfriendly adjuvants (additives), such as squalene, mercury, aluminium, formaldehyde and other un-welcomed guests. Vaccine makers add some of these to provoke the immune response, however the data I have seen puts its users at risk of irrevocable harm.
. J Clin Virol. 2011 Mar;50(3):194-200. Epub 2011 Jan 15.
. Exp Biol Med (Maywood). 2010 Dec;235(12):1395-6; discussion 1397.
. J Manipulative Physiol Ther. 1999 Oct;22(8):530-3
. The Journal of Orthomolecular Medicine Vol. 13, 4th Quarter 1998
. Southern Med Surg, 1948; 110/2: 36-38, 46.
. Journal of Orthomolecular Medicine Vol. 22, No. 1, 200
. Cathcart RF. Treatment of the flu with massive doses of vitamin C.
. 2008, Vol. 17, No. 3 , Pages 169-177 (doi:10.1080/13590840802305423)
. J Nutr. 2000 May;130(5):1132-9.
. Cochrane Collaboration Database of Systemic Reviews. 2006(3)
. Consumer Health Digest – “Japan’s Greatest Lesson: Compulsory Flu Vaccine Reveals No Benefit and Deadly Side Effects”