Influenza is a viral infection of the lungs. There are many symptoms:
- body aches and muscle pain
- dry cough
- runny nose
- dry or sore throat
The lining around the lungs may become inflamed. This can lead to bacterial pneumonia.
Influenza is most common in winter, a time when:
- Solar ultraviolet-B (UVB) doses are low.
- The weather is cold. This prevents white blood cells from reaching the lining of the respiratory tract and fighting the virus.
- The humidity is low. Dry air allows the virus to live longer outside of the body.
Sunlight exposure and influenza risk
Influenza rates peak in winter. There is less solar UVB light in winter, especially in areas farther from the equator. Thus, vitamin D levels are at their lowest.
Vitamin D and influenza
Vitamin D from sunlight or supplements reduces the risk of influenza.
Two randomized controlled trials found reduced incidence of influenza for those taking higher doses of vitamin D. A study involving African-American postmenopausal women in New York found a 60% reduced risk of colds and influenza for those taking 800 IU/d vitamin D3 and 90% reduced risk for those taking 2000 IU/d.
Another study in Japan, involving school children taking 1200 IU/d vitamin D3 vs. 200 IU/d, found a 67% reduction in Type A influenza, but no effect for Type B influenza. Type A influenza includes H1N1 varieties, which was the type involved in the 1918-19 pandemic influenza and the 2009 “swine flu” infections.
According to an observational study, vitamin D provides protection against influenza. This occurs when vitamin D levels in the blood are more than 38 ng/mL (95 nmol/L).
How vitamin D works
To enhance the body’s immune system, vitamin D:
- Produces cathelicidin and defensins—These proteins have antiviral effects to combat viruses.
- Reduces inflammation—As a result, body temperature does not rise as much, and the lining of the lungs is less disturbed. This makes it harder for bacteria to give rise to pneumonia.
High levels of vitamin D may prevent or lower the risk of influenza. Vitamin D may also reduce symptoms of influenza and reduce the risk of developing pneumonia following influenza. Vaccines strengthen the body’s ability to fight infection. Therefore, combining high levels of vitamin D and anti-influenza vaccines provide the best protection.
Based on several studies, raising vitamin D blood levels to 40 ng/ml (100 nmol/l) may reduce the risk of influenza. For most people, this involves taking 1000–5000 international units (IU) (25–125 mcg)/day of vitamin D during the influenza season.
On average, 2000-5000 IU/day vitamin D3 may provide protection against influenza. Vitamin D3, the true form of vitamin D, is produced in the skin. Larger doses of vitamin D taken for a short time strengthen the immune system. This allows the body to fight infection.
This evidence summary was written by:
William B. Grant, Ph.D.
Sunlight, Nutrition, and Health Research Center (SUNARC)
P.O. Box 641603
San Francisco, CA 94164-1603, USA
The summary was reviewed by:
- Adrian Gombart <Adrian.Gombart@oregonstate.edu>
Complete bibliography of influenza research used in this summary
The research we have cited in our summary is listed below, with links to PubMed abstracts and full-text for those who wish to explore further.
- Aloia, J. F. Li-Ng, M. Re: epidemic influenza and vitamin D. Epidemiol Infect. 2007 Oct; 135 (7): 1095-6; author reply 1097-8.
- Ardizzone, S. Cassinotti, A. Trabattoni, D. Manzionna, G. Rainone, V. Bevilacqua, M. Massari, A. Manes, G. Maconi, G. Clerici, M. Bianchi Porro, G. Immunomodulatory effects of 1,25-dihydroxyvitamin D3 on TH1/TH2 cytokines in inflammatory bowel disease: an in vitro study. Int J Immunopathol Pharmacol. 2009 Jan-Mar; 22 (1): 63-71.
- Banajeh, S. M. Nutritional rickets and vitamin D deficiency–association with the outcomes of childhood very severe pneumonia: a prospective cohort study. Pediatr Pulmonol. 2009 Dec; 44 (12): 1207-15.
- Britten, R. H. The incidence of epidemic influenza, 1918-19. Pub Health Rep. 1932; 47303-39.
- Cannell, J. J. Vieth, R. Umhau, J. C. Holick, M. F. Grant, W. B. Madronich, S. Garland, C. F. Giovannucci, E. Epidemic influenza and vitamin D. Epidemiol Infect. 2006 Dec; 134 (6): 1129-40.
- Cannell, J. J. Zasloff, M. Garland, C. F. Scragg, R. Giovannucci, E. On the epidemiology of influenza. Virol J. 2008 Feb; 25 (5): 29.
- Chan, M. C. Cheung, C. Y. Chui, W. H. Tsao, S. W. Nicholls, J. M. Chan, Y. O. Chan, R. W. Long, H. T. Poon, L. L. Guan, Y. Peiris, J. S. Proinflammatory cytokine responses induced by influenza A (H5N1) viruses in primary human alveolar and bronchial epithelial cells. Respir Res. 2005 Nov; 11 (6): 135.
- Cheung, C. Y. Poon, L. L. Lau, A. S. Luk, W. Lau, Y. L. Shortridge, K. F. Gordon, S. Guan, Y. Peiris, J. S. Induction of proinflammatory cytokines in human macrophages by influenza A (H5N1) viruses: a mechanism for the unusual severity of human disease?. Lancet. 2002 Dec 7; 360 (9348): 1831-7.
- Creanga, A. A. Johnson, T. F. Graitcer, S. B. Hartman, L. K. Al-Samarrai, T. Schwarz, A. G. Chu, S. Y. Sackoff, J. E. Jamieson, D. J. Fine, A. D. Shapiro-Mendoza, C. K. Jones, L. E. Uyeki, T. M. Balter, S. Bish, C. L. Finelli, L. Honein, M. A. Severity of 2009 pandemic influenza A (H1N1) virus infection in pregnant women. Obstet Gynecol. 2010 Apr; 115 (4): 717-26.
- du Prel, J. B. Puppe, W. Grondahl, B. Knuf, M. Weigl, J. A. Schaaff, F. Schmitt, H. J. Are meteorological parameters associated with acute respiratory tract infections?. Clin Infect Dis. 2009 Sep 15; 49 (6): 861-8.
- Ginde, A. A. Liu, M. C. Camargo, C. A., Jr. Demographic differences and trends of vitamin D insufficiency in the US population, 1988-2004. Arch Intern Med. 2009 Mar 23; 169 (6): 626-32.
- Gombart, A. F. The vitamin D-antimicrobial peptide pathway and its role in protection against infection. Future Microbiol. 2009 Nov; 41151-65.
- Grant, W. B. Cannell, J. J. Pregnant women are at increased risk for severe A influenza because they have low serum 25-hydroxyvitamin D levels. Crit Care Med. 2010 Sep; 38 (9): 1921; author reply 1921-2.
- Grant, W. B. Giovannucci, E. 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. Dermato-Endocrinology. 2009; 1 (4): 215-219.
- Hansdottir, S. Monick, M. M. Hinde, S. L. Lovan, N. Look, D. C. Hunninghake, G. W. Respiratory epithelial cells convert inactive vitamin D to its active form: potential effects on host defense. J Immunol. 2008 Nov 15; 181 (10): 7090-9.
- Hansdottir, S. Monick, M. M. Lovan, N. Powers, L. Gerke, A. Hunninghake, G. W. Vitamin D decreases respiratory syncytial virus induction of NF-kappaB-linked chemokines and cytokines in airway epithelium while maintaining the antiviral state. J Immunol. 2010 Jan 15; 184 (2): 965-74.
- Hanslik, T. Boelle, P. Y. Flahault, A. Preliminary estimation of risk factors for admission to intensive care units and for death in patients infected with A(H1N1)2009 influenza virus, France, 2009-2010. PLoS Curr Influenza. 2010 Mar 9; RRN1150.
- Harris, S. S. Dawson-Hughes, B. Seasonal changes in plasma 25-hydroxyvitamin D concentrations of young American black and white women. Am J Clin Nutr. 1998 Jun; 67 (6): 1232-6.
- Heaney, R. P. Davies, K. M. Chen, T. C. Holick, M. F. Barger-Lux, M. J. Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr. 2003 Jan; 77 (1): 204-10.
- Hope-Simpson, R. E. The role of season in the epidemiology of influenza. J Hyg (Lond). 1981 Feb; 86 (1): 35-47.
- Hypponen, E. Power, C. Hypovitaminosis D in British adults at age 45 y: nationwide cohort study of dietary and lifestyle predictors. Am J Clin Nutr. 2007 Mar; 85 (3): 860-8.
- La Ruche, G. Tarantola, A. Barboza, P. Vaillant, L. Gueguen, J. Gastellu-Etchegorry, M. The 2009 pandemic H1N1 influenza and indigenous populations of the Americas and the Pacific. Euro Surveill. 2009 Oct; 14 (42):
- Liu, P. T. Stenger, S. Tang, D. H. Modlin, R. L. Cutting edge: vitamin D-mediated human antimicrobial activity against Mycobacterium tuberculosis is dependent on the induction of cathelicidin. J Immunol. 2007 Aug 15; 179 (4): 2060-3.
- McNally, J. D. Leis, K. Matheson, L. A. Karuananyake, C. Sankaran, K. Rosenberg, A. M. Vitamin D deficiency in young children with severe acute lower respiratory infection. Pediatr Pulmonol. 2009 Oct; 44 (10): 981-8.
- Mookherjee, N. Rehaume, L. M. Hancock, R. E. Cathelicidins and functional analogues as antisepsis molecules. Expert Opin Ther Targets. 2007 Aug; 11 (8): 993-1004.
- Ross, T. et al Seroprevalence Following the Second Wave of Pandemic 2009 H1N1 Influenza. PLoS Curr Influenza. 2010, Feb; 24
- Sabetta, J. R. DePetrillo, P. Cipriani, R. J. Smardin, J. Burns, L. A. Landry, M. L. Serum 25-hydroxyvitamin d and the incidence of acute viral respiratory tract infections in healthy adults. PLoS One. 2010 June; 5 (6): e11088.
- Shaman, J. Kohn, M. Absolute humidity modulates influenza survival, transmission, and seasonality. Proc Natl Acad Sci U S A. 2009 Mar 3; 106 (9): 3243-8.
- Shaman, J. Pitzer, V. Viboud, C. Lipsitch, M. Grenfell, B. Absolute humidity and the seasonal onset of influenza in the continental US. PLoS Curr Influenza. 2009 Dec; 18RRN1138.
- Urashima, M. Segawa, T. Okazaki, M. Kurihara, M. Wada, Y. Ida, H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010 May; 91 (5): 1255-60.
- Writing Committee of the WHO Consultation on Clinical Aspects of Pandemic (H1N1) 2009 Influenza, Medical progress: Clinical aspects of pandemic 2009 influenza A (H1N1) virus infection. N Engl J Med. 2010; 362 (May 6): 1708-19.
- Yamshchikov, A. V. Desai, N. S. Blumberg, H. M. Ziegler, T. R. Tangpricha, V. Vitamin D for treatment and prevention of infectious diseases: a systematic review of randomized controlled trials. Endocr Pract. 2009 Jul-Aug; 15 (5): 438-49.