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Information on the latest vitamin D news and research.

Find out more information on deficiency, supplementation, sun exposure, and how vitamin D relates to your health.

Influenza

woman with tissue

What is influenza?

Influenza, also known as the flu, is a contagious respiratory infection that is caused by the influenza virus and infects the nose, throat and lungs. In the United States, the peak of flu season is generally from late November through March, but can last through May. Young children, the elderly and people with chronic diseases are more likely to have severe complications from developing the flu. Influenza can cause hospitalization or death in serious cases.[1] In 1918 the flu killed 50,000,000 people and there is nothing, including flu shots, to keep that from happening again. It is just a matter of time.

Since the common influenza virus changes every season, so do the vaccines but the vaccines are of marginable effectiveness. The severity and prevalence of the flu changes each season . The flu shot is a guess about which 3 strains are likely to be active in any one year and the experts often guess incorrectly.

The three types of influenza are types A, B and C. Types A and B are the viruses that are typically responsible for seasonal influenza epidemics. Type C influenza viruses can cause a mild respiratory infection, but this virus generally does not cause flu epidemics. There are also subtypes of type A influenza viruses, which are based on 2 proteins found on the virus (abbreviated H and N). A common example is H1N1. The most common influenza virus in the 2012-2013 flu season was H3N21.

What are the symptoms of influenza?

Someone who is infected with the influenza virus may experience some or all of these symptoms:[2]

  • Fever or chills
  • Cough
  • Sore throat
  • Muscle, body and head aches
  • Fatigue
  • Vomiting and diarrhea, which is more common in children with influenza.
  • Prostration, which is the desire to lie down.

Most of the time, a healthy person will recover from influenza in a few days to less than 2 weeks.  Sometimes, people can develop serious complications from influenza, like pneumonia, bronchitis, or sinus and ear infections. Influenza can also make a chronic condition worse, like asthma. When influenza is fatal, oftentimes it is due to a bacterial infection, like pneumonia, that is secondary to the influenza.[3]

What are the risk factors for influenza?

Annually, influenza affects about one-fifth of the U.S. population.[4] For the 2012-2013 flu season, more than half of hospitalizations related to influenza were in adults over the age of 65.

  • On average, about 10% of the U.S. population will get the flu each year.[5] However, during the AH1N1 pandemic of 2009-10, the number of cases rose to 95,000, or 20%.[6]
  • About 28,000 adults are hospitalized each year because of flu complications in the US.[7]
  • The number of people who die each year from flu-related causes in the U.S. ranges from 3,000 to 49,000 but some experts think that excess wintertime deaths are due to the flu..

People who are at higher risk for developing influenza include:[8]

  • People over age 65
  • Children, especially those under age 2
  • People with chronic disease, like asthma, diabetes, lung or heart disease
  • People with weakened immune systems, like those with cancer or HIV/AIDS
  • People who are morbidly obese
  • Pregnant women
  • American Indians and Alaskan Natives

Note that many people in those categories have low vitamin D levels.

How does influenza spread?

Someone who is infected with the virus can be contagious 1 day before influenza symptoms develop, and up to 7 days after they become sick. Young children or people with compromised immune systems are able to infect others for a longer period of time.[9]

It is widely thought that the influenza virus can spread when someone who is infected coughs, sneezes or speaks but this has never been proven. Droplets containing the influenza virus are thought to travel through the air and can be directly inhaled by another person through the mouth or nose. The droplets may also land on a surface and could be picked up and infecting the individual after touching their eyes, nose or mouth.[10]

The peak influenza season is near the end of the year in the northern hemisphere, a time when it is coldest and the air is driest. This climate allows the influenza virus to live outside of the human body for an extended period of time.[11]

Once an individual has had influenza, their body creates antibodies to help protect against that specific virus in the future. However, because the influenza viruses changes every year, these antibodies won’t fight off the newer virus. Vaccines help the body to make antibodies against the most likely current influenza virus.[12]

What is the link between influenza and vitamin D?

Vitamin D is an important factor in immune system health. Some studies have shown that there is a link between vitamin D status and the risk of developing influenza. People who have low vitamin D levels may have an increased risk of developing influenza.[13]

Vitamin D receptors are found on the surface of a cell where they receive chemical signals. By attaching themselves to a receptor, these chemical signals direct a cell to do something, for example, to act in a certain way, or to divide or die.

There are vitamin D receptors found on cells in the immune system, and vitamin D can bind to these receptors.[14] Vitamin D works in the immune system by reducing levels of inflammatory proteins called cytokines, as well as increasing the amount of antimicrobial proteins, which are naturally occurring antibiotics that destroy invading germs and viruses. This combination of lowering inflammation and increasing antimicrobial defenses can help an individual’s immune system fight infections more effectively.[15],[16] These actions also reduce the risk of developing pneumonia, which is the primary complication of influenza resulting in mortality.[17]

There are two branches of the immune system: adaptive and innate. The adaptive immune system develops based on previous exposure to a virus. The innate immune system responds quickly to foreign invaders, and the effectiveness is determined by the levels of immune cells and proteins an individual has.[18] Low vitamin D levels may result in some parts of the innate immune system not functioning as well.[19]

Influenza epidemics occur in the winter. Vitamin D levels in the greater population are dramatically lower in the winter as well.[20] Since influenza is seasonal, it is thought that vitamin D might be a factor that can affect someone’s chances of getting the flu, as first proposed by John Cannell in 2006.[21]

What does the research say in general about influenza and vitamin D?

Preventing influenza

Most studies that have been conducted regarding influenza have shown that people who have lower levels of vitamin D are more likely to get the flu. One study looked at the levels of vitamin D in people with prostate cancer and their immune response to the influenza vaccine. This study found that people who had higher levels of vitamin D had an improved response to the vaccine, meaning that they would be more protected against getting influenza.[22] However, another study which evaluated vitamin D and influenza in people over age 50 found that vitamin D levels didn’t have a significant effect on their immune response to the influenza vaccine.[23]

A study comprised of healthy adults found that people with lower levels of vitamin D were twice as likely to develop influenza, compared to people with high levels of vitamin D.[24]

Clinical trials are needed to determine whether or not supplementing with vitamin D may help protect against getting influenza.

Treating and recovering from influenza

Not many studies have been conducted evaluating the treatment effect of vitamin D for influenza, but some research has shown a relationship between vitamin D status and the duration of the influenza infection. Other studies have looked at influenza outcomes, like pneumonia or death, in large influenza epidemics that happened in the past.

A study that looked at the deaths associated with the large 1918-1919 influenza pandemic found that the lowest influenza-related death rates in the United States were found in the city with the highest amounts of UVB light, which helps the body make vitamin D. They also found that the most influenza deaths were in the city with the lowest amounts of UVB radiation.[25]

People with enough vitamin D may recover from influenza faster than people with low levels of vitamin D. A study found that people with vitamin D levels above 38 ng/mL recovered from influenza in an average of 2 days; whereas people with vitamin D levels below 38 ng/mL took an average of 9 days to recover from influenza.[26]

What does recent research say about vitamin D and influenza?

An experiment conducted in the United States gave elderly African-American women either 800 IU vitamin D per day for 2 years, then 2,000 IU per day for the 3rd year, or a dummy pill. The researchers looked at how many times those women got influenza over the 3 years. They found that:[27]

  • The vitamin D group had fewer influenza symptoms compared to the dummy pill group.
  • Only one person in the vitamin D group had influenza when the dose was at 2,000 IU per day for one year. However, there were 30 cases of influenza or colds in the three years for those taking the dummy pill.
  • The dummy pill group had influenza symptoms mostly in the winter, whereas the people who got infected in the vitamin D group had symptoms, independent of the season.

This experiment suggests that vitamin D, especially at higher doses, may help to protect against seasonal influenza. The researchers concluded that vitamin D supplements might be useful to prevent the flu, but more experiments are needed to confirm this.

An experiment evaluated the effects of vitamin D supplementation on influenza risk in Japanese school children. The researchers gave children either 1,200 IU vitamin D per day for 3 months during the winter, or a dummy pill. They found that:[28]

  • More children in the dummy pill group got influenza A than children in the vitamin D group.
  • There was a preventive effect of 1,200 IU vitamin D per day on children getting influenza A.

The researchers conclude that taking 1,200 IU of vitamin D in children can help to protect against seasonal influenza A. In this study, there was no effect of vitamin D on influenza B, possibly because vitamin D may respond in different ways to the inflammatory proteins in the viruses.

A study conducted in 2011 looked at vitamin D levels and respiratory infections, like influenza, in a large group of British adults. The researchers found that:[29]

  • For each 4 ng/ml increase in vitamin D levels in the body, there was a 7% lower chance of developing influenza.
  • There was a seasonal pattern of influenza, which was the same as the seasonal pattern of vitamin D levels.
  • Influenza infections decreased when vitamin D levels increased. However, since this study was observational, the researchers couldn’t conclude for certain if higher vitamin D levels protected against the flu.

Key points from the research

  • People who get influenza are more likely to have low levels of vitamin D.
  • Vitamin D can help reduce inflammation caused by the influenza virus and increase the number of antimicrobial proteins that fight against viruses.
  • Influenza infections increase during the winter, which is when vitamin D levels are known to decrease in the population.
  • Some experiments have shown that taking vitamin D supplements can reduce the chances of getting influenza.
  • Having high levels of vitamin D may help decrease recovery time from an influenza infection.
  • Some researchers recommend higher vitamin D levels to protect against influenza.

What does this mean for me?

When the winter flu season arrives, be sure to take vitamin D supplements to maintain a vitamin D status between 40-80 ng/ml (100-200 nmol/l).

If get influenza, you can safely take 50,000 IU/day for 5 days to try and fight off the infection. However, vitamin D shouldn’t take the place of your treatment medications. Talk to your physician for more advice about taking supplements.

References

[1] Grant, W.B. and E. Giovannucci, The possible roles of solar ultraviolet-B radiation and vitamin D in reducing case-fatality rates from the 1918-1919 influenza pandemic in the United States. Dermatoendocrinol, 2009. 1(4): p. 215-9.

[2] The Centers for Disease Control and Prevention. Seasonal Influenza: Flu Basics, 2015.

[3] Grant, W.B. and E. Giovannucci, The possible roles of solar ultraviolet-B radiation and vitamin D in reducing case-fatality rates from the 1918-1919 influenza pandemic in the United States. Dermatoendocrinol, 2009. 1(4): p. 215-9

[4] Rees, J.R., Hendricks, K., Barry, E.L.,  et al., Vitamin D3 supplementation and upper respiratory tract infections in a randomized, controlled trial. Clin Infect Dis, 2013. 57(10): p. 1384-92.

[5] Molinari, N.A., Ortega-Sanchez, I.R., Messonnier, M.L.,  et al., The annual impact of seasonal influenza in the US: measuring disease burden and costs. Vaccine, 2007. 25(27): p. 5086-96.

[6] Shrestha, S.S., Swerdlow, D.L., Borse, R.H.,et al., Estimating the burden of 2009 pandemic influenza A (H1N1) in the United States (April 2009-April 2010). Clin Infect Dis, 2011. 52 Suppl 1: p. S75-82.

[7] Ortiz, J.R., Neuzil, K.M., Shay, D.K., et al., The burden of influenza-associated critical illness hospitalizations. Crit Care Med, 2014. 42(11): p. 2325-32.

[8] The Centers for Disease Control and Prevention. Seasonal Influenza: Flu Basics, 2015.

[9] The Centers for Disease Control and Prevention. Seasonal Influenza: Flu Basics, 2015.

[10] Mayo Clinic. Diseases and Conditions: Influenza (flu), 2015.

[11] Shaman, J., Jeon, C.Y., Giovannucci, E., Lipsitch, M., Shortcomings of vitamin D-based model simulations of seasonal influenza. PLoS One, 2011. 6(6): p. e20743.

[12] Mayo Clinic. Diseases and Conditions: Influenza (flu), 2015.

[13] Laaksi, I., Ruohola, J.P., Tuohimaa, P., et al., An association of serum vitamin D concentrations < 40 nmol/L with acute respiratory tract infection in young Finnish men. Am J Clin Nutr, 2007. 86(3): p. 714-7.

[14] Lang, P.O. and D. Samaras, Aging adults and seasonal influenza: does the vitamin d status (h)arm the body? J Aging Res, 2012. 2012: p. 806198.

[15] Cannell, J.J., Vieth, R., Umhau, J.C., et al., Epidemic influenza and vitamin D. Epidemiol Infect, 2006. 134(6): p. 1129-40.

[16] Sundaram, M.E., McClure, D.L., VanWormer, J.J., et al., Influenza vaccination is not associated with detection of noninfluenza respiratory viruses in seasonal studies of influenza vaccine effectiveness. Clin Infect Dis, 2013. 57(6): p. 789-93.

[17] Grant, W.B. and E. Giovannucci, The possible roles of solar ultraviolet-B radiation and vitamin D in reducing case-fatality rates from the 1918-1919 influenza pandemic in the United States. Dermatoendocrinol, 2009. 1(4): p. 215-9.

[18] Cannell, J.J., Zasloff, M., Garland, C.F., et al., On the epidemiology of influenza. Virol J, 2008. 5: p. 29.

[19] Yusupov, E., Li-Ng, M., Pollack, S., et al., Vitamin d and serum cytokines in a randomized clinical trial. Int J Endocrinol, 2010. 2010.

[20] Hypponen, E. and C. Power, Hypovitaminosis D in British adults at age 45 y: nationwide cohort study of dietary and lifestyle predictors. Am J Clin Nutr, 2007. 85(3): p. 860-8.

[21] Cannell, J.J., Vieth, R., Umhau, J.C., et al., Epidemic influenza and vitamin D. Epidemiol Infect, 2006. 134(6): p. 1129-40.

[22] Chadha, M.K., Fakih, M., Muindi, J., et al., Effect of 25-hydroxyvitamin D status on serological response to influenza vaccine in prostate cancer patients. Prostate, 2011. 71(4): p. 368-72.

[23] Sundaram, M.E., McClure, D.L., VanWormer, J.J., et al., Influenza vaccination is not associated with detection of noninfluenza respiratory viruses in seasonal studies of influenza vaccine effectiveness. Clin Infect Dis, 2013. 57(6): p. 789-93.

[24] Sabetta, J.R., DePetrillo, P., Cipriani, R.J., et al., Serum 25-hydroxyvitamin d and the incidence of acute viral respiratory tract infections in healthy adults. PLoS One, 2010. 5(6): p. e11088.

[25] Grant, W.B. and E. Giovannucci, The possible roles of solar ultraviolet-B radiation and vitamin D in reducing case-fatality rates from the 1918-1919 influenza pandemic in the United States. Dermatoendocrinol, 2009. 1(4): p. 215-9.

[26] Cannell, J.J., Vieth, R., Umhau, J.C., et al., Epidemic influenza and vitamin D. Epidemiol Infect, 200 Aloia, J.F. and M. Li-Ng, Re: epidemic influenza and vitamin D. Epidemiol Infect, 2007. 135(7): p. 1095-6; author reply 1097-8.. 134(6): p. 1129-40.

[27] Sundaram, M.E., McClure, D.L., VanWormer, J.J., et al., Influenza vaccination is not associated with detection of noninfluenza respiratory viruses in seasonal studies of influenza vaccine effectiveness. Clin Infect Dis, 2013. 57(6): p. 789-93.

[28] Urashima, M., Segawa, T., Okazaki, M., et al., Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr, 2010. 91(5): p. 1255-60.

[29] Berry, D.J., Hesketh, K., Power, C., Hyppönen, E., Vitamin D status has a linear association with seasonal infections and lung function in British adults. Br J Nutr, 2011. 106(9): p. 1433-40.

This page was last updated December 2015.