There are many hypotheses as to why seasonal colds are so widespread only during a certain season: low temperatures, dry air, crowding together indoor in the winter, travelling patterns, seasonality of ultraviolet (UV) radiation from the sun that might kill pathogens, circannual rhythms of hormones like the “dark hormone” melatonin, etc. Possibly, the seasonality of viruses and batteries is also partly responsible.
In this study the authors have studied the annual death numbers of influenza and pneumonia in Norway for the time period 1980-2000 and compared with UVB level and vitamin D level produced by it.
Influenza-related mortality is increased 2 months after reaching the minimum vitamin D level. This, however, may be caused by the fact that the disease usually starts weeks prior to the point of death, when the immune system is at the lowest.
The vitamin D level is maximal close to midsummer; the lowest vitamin D is found in February, and average level as low as 25 nmol/l have been observed among women avoiding direct sun exposure.
Norway is localized from 58 to 70 degrees north, vitamin D being synthesized in skin exclusively in the summer months. Generally, there is no distinct seasonal pattern in the tropics. At latitudes between 20 and 30 degrees north, clear winter seasons of influenza are found. This is at first sight surprising. However, at 25 degrees north the rate of vitamin D synthesis in human skin is about five times larger in late June than in late December.
It is common knowledge that vitamin D strengthens immune protection. 
Solar radiation triggers robust seasonal vitamin D production in the skin; vitamin D deficiency is common in the winter. The elderly only make about 25% of the vitamin D compared to 20-year-olds after exposure to the same amount of sunlight.
No vitamin D is made in the skin at latitude 52° N (the latitude of London) from about October to March because atmospheric ozone easily filters out UVB radiation. The average excess winter mortality in Great Britain alone is inversely related to hours of sunlight. Every additional hour of sunlight reduces mortality by 2·9%
People who received a live influenza vaccine in February experienced high fever twice as often as those vaccinated in June; the lowest rate of antibody formation is registered in summer. 40% of people vaccinated in December shed the virus, compared to 16% of those vaccinated in June.
There was no evidence of toxicity in young men taking 50 000 IU of vitamin D a day for 6 weeks (although such a dose would be toxic if taken over a longer period). A single dose of 600,000 МЕ taken via injection did not show any toxicity either.
A randomized, double-blind, placebo-controlled study of the effect of vitamin D on the incidence of influenza.
Influenza occurred 42% less in schoolchildren who received 1200 МЕ of vitamin D3 supplements daily.
Systematic review and meta-analysis of the effect of vitamin D on the incidence of acute respiratory disease. 5 randomized placebo-controlled trials were included. Vitamin D reduces the risk of influenza by 42% in children and by 35% in adults.Vitamin D and Respiratory Tract Infections: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. 2013, Bergman, PLoS One
A similar review and meta-analysis (11 studies). Vitamin D reduces the risk of respiratory tract infections (RTI) by 36%. Small daily doses reduce the risk of RTI by 49%, whereas infrequent large doses of vitamin D are less efficient and reduce the risk of RTI only by 14%.Vitamin D, innate immunity and outcomes in community acquired pneumonia. 2011, Leow, Respirology
Among patients admitted to the hospital with pneumonia, mortality in those who had a vitamin D deficiency (less than 30 nmol/L) was 12.7 times higher than among those who had a vitamin level higher than 50 nmol/L.
Vitamin D level in newborns, hospitalized with acute lower respiratory infection, was significantly lower than that in healthy newborns (9.12 ng/mL vs 16.33 ng/mL).
Vitamin D level in mothers of the hospitalized newborns was also lower than that of the mothers of healthy newborns (13.38 ng/mL vs 22.79 ng/mL).
Authors observed a strong positive correlation between newborns' and mothers' level of vitamin D.
Authors measured the vitamin D level in the umbilical cord of 922 newborn. During the 3 months following birth the risk of respiratory tract infections in newborns with vitamin D level < 25 nmol/L was twice as high as that in newborns with vitamin D level > 75 nmol/L. Risk of wheezing in children with low vitamin D at birth was twice as high till they were 5 years old than that of newborns with high vitamin D at birth.Cord blood vitamin D deficiency is associated with respiratory syncytial virus bronchiolitis. 2011, Belderbos, Pediatrics
Newborns with vitamin D level in the umbilical cord below 50 nmol/L had severe lower respiratory tract infection during the first year of their lives 6.2 times more often than those whose vitamin D was above 75 nmol/L.
In western countries 40% of pregnant women and 50% newborns have vitamin D deficiency. This deficiency during pregnancy is associated with type 1 diabetes, multiple sclerosis, schizophrenia and respiratory tract infections in children.
Children with vitamin D >22.5 nmol/l contracted severe acute lower respiratory infection (ALRI) 91% less frequently. Exclusive breastfeeding in the first 4 months of life reduced the risk of ALRI by 58%.Serum 25-hydroxyvitamin D, mortality, and incident cardiovascular disease, respiratory disease, cancers, and fractures: a 13-y prospective population study. 2014, Khaw, Am J Clin Nutr
A prospective study of 14 thousand men and women living in the UK, lasting 13 years.
The level of vitamin D appears to be in inverse correlation with mortality. Among those whose vitamin D level was >90 nmol/L, mortality was 34% lower than among subjects with vitamin D <30 nmol/L.
Risk of death from cardiovascular diseases in these subjects was 38% lower, from cancer – 15% lower and from lower respiratory infections – 78% lower.
Young men with low level of vitamin D (<40 nmol/L) had acute respiratory tract infections 63% more often than young men with level of vitamin D >40 nmol/L.
Smokers had lower vitamin D than non-smokers. Young men who did physical exercise more than 5 hours per week had a higher vitamin D level.
Adults with vitamin D <38 ng/mL suffered from acute viral respiratory tract infections 2.7 times less, and the duration of the disease was 4.9 times less. Influenza vaccine did not affect the incidence of acute viral respiratory tract infections.On the epidemiology of influenza. 2008, Cannell, Virol J
A very interesting review on epidemiology of influenza and vitamin D, well worth reading. It turns out that the numerous laboratory experiments failed to prove that a person with flu can infect a healthy person.Vitamin D: a new anti-infective agent? 2014, Borella, Ann N Y Acad Sci
An association has been established between low levels of vitamin D and upper respiratory and enteric infections, pneumonia, otitis, Clostridium infections, vaginosis, urinary tract infections, sepsis, influenza, dengue, hepatitis B, hepatitis C, and HIV infections. Vitamin D is used in treatment of osteoporosis, rheumatoid arthritis and cancer.
Till mid 20-th century, treatment of tuberculosis patients was restricted to sun exposure in sanatoria and fish oil. These two methods have one thing in common – vitamin D.
Tamiflu and Relenza are useless drugs which do more harm than good. The author writes that in nursing homes he has seen some of the patients and staff develop vomiting; some with serious diarrhea (also a known side effect); some with acute confusion, hallucinations, or delirium; and a number with worsening cognitive function. These medications should no longer be stockpiled or used. This would result in tremendous savings in health care dollars.
The Institute of Medicine recommendation for adults younger than 70 years of age is 600 IU of vitamin D daily, to achieve a level of 50 nmol/L in more than 97.5% of individuals. Regrettably, a statistical error has resulted in erroneous recommendations by the Institute of Medicine leading to this conclusion. In actual fact one needs to take 8800 IU of vitamin D to achieve this level in 97.5% of the population.
The author and his colleague have introduced vitamin D at doses that have achieved greater than 100 nmol/L in most of their patients for the past number of years, and they now see very few patients with the flu or influenza like illness. In those patients who do have influenza, they have treated them with the vitamin D hammer: a 1-time 50 000 IU dose of vitamin D3 or 10 000 IU 3 times daily for 2 to 3 days. The results are dramatic, with complete resolution of symptoms in 48 to 72 hours. The cost of vitamin D is about a penny for 1000 IU, so this treatment costs less than a dollar.
Some health-care workers may be hesitant to take a vaccine because it contains a mercury preservative-thimerosal - which can be harmful to their health. These health-care workers and patients should be tested and treated for vitamin D deficiency to prevent exacerbation of a respiratory infection.
Optimal vitamin D level is 50-75 ng/mL, and an optimal dose for grown-ups is 4000-5000 IU daily.