Geographical variations in solar ultraviolet-B doses may explain some of the spatial-temporal excess mortality patterns of the 1918-1919 influenza pandemic in Spain William B. Grant, Sunlight, Nutrition, and Health Research Center 19 February 2015 The paper by Chowell and colleagues presents maps of excess respiratory death rates during the 1918-1919 influenza pandemic in Spain . The summer and winter peak rates are in the center and south of Spain, while the fall peak rates are predominantly in the north. The authors suggested that colder temperatures and lower humidity in the north and in fall/winter might explain the observed patterns. I agree that this suggestion is generally correct. However, there is another factor that should also be considered - the role of solar ultraviolet B (UVB) irradiance and vitamin D production. In an ecological study of the 1918-1919 influenza pandemic in the United States, it was reported that both summertime and wintertime solar UVB doses were inversely correlated with case-fatality rates for a dozen communities . Most of the deaths associated with the influenza were due to pneumonia, and occurred about ten days after the start of influenza. The mechanisms suggested were induction of cathelicidin, which has antibiotic properties, and reduction of the cytokine storm due to the body fighting influenza. The UVB-vitamin D-influenza hypothesis was proposed in 2006 by John Cannell and colleagues . This hypothesis has gained further support from vitamin D randomized controlled trials involving people with low 25-hydroxyvitamin D [25(OH)D] concentrations at the time of enrollment in the trials [4-6]. Broadband solar UVB doses have been measured for 14 locations in Spain . In summer, values at solar noon ranged from 1234 to 1726 units but do not follow a strict latitude gradient. In winter, the values ranged from 160 to 348 and showed a strong latitudinal gradient. Based on these values, the higher mortality rates in winter and in the north may be due in part to lower UVB doses in winter. The higher rates in south and central Spain in summer may be related to where influenza was first introduced into Spain. A related paper found that cancer mortality rates were often inversely correlated with latitude and directly correlated with nonmelanoma skin cancer . This analysis suggests that 25(OH)D concentrations be raised by vitamin D supplementation in winter in Europe to reduce the risk of respiratory infections. Optimal 25(OH)D concentrations are between 75 and 125 nmol/L , which can be achieved with 800-2000 IU/d vitamin D3 for most people . There are also many other beneficial effects of higher 25(OH)D concentrations [9, 11]. References 1. Chowell G, Erkoreka A, Viboud C, Echeverri-Dávila B. Spatial-temporal excess mortality patterns of the 1918-1919 influenza pandemic in Spain. BMC Infect Dis. 2014;14:371. 2. Grant WB, 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. Dermatoendocrinol. 2009;1(4):215-9. 3. Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, Madronich S, Garland CF, Giovannucci E. Epidemic influenza and vitamin D. Epidemiol Infect. 2006;134(6):1129-40. 4. Aloia JF, Li-Ng M. Re: epidemic influenza and vitamin D. Epidemiol Infect. 2007 Oct;135(7):1095-6; author reply 1097-8. 5. 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;91(5):1255-60. 6. Camargo CA Jr, Ganmaa D, Frazier AL, Kirchberg FF, Stuart JJ, Kleinman K, Sumberzul N, Rich-Edwards JW. Randomized trial of vitamin D supplementation and risk of acute respiratory infection in Mongolia. Pediatrics. 2012;130(3):e561-7. 7. Martínez-Lozano JA, Utrillas, MP, Nunez JA, Esteve AR, Gomea-Amo JL, Estelles V, Pedros R. Measurement and Analysis of Broadband UVB Solar Radiation in Spain. Photochemistry and Photobiology, 2012;88:1489¿1496 8. Grant WB. An ecologic study of cancer mortality rates in Spain with respect to indices of solar UV irradiance and smoking. Int J Cancer. 2007;120(5):1123-7. 9. Pludowski P, Holick MF, Pilz S, Wagner CL, Hollis BW, Grant WB, Shoenfeld Y, Lerchbaum E, Llewellyn DJ, Kienreich K, Soni M. Vitamin D effects on musculoskeletal health, immunity, autoimmunity, cardiovascular disease, cancer, fertility, pregnancy, dementia and mortality- a review of recent evidence. Autoimmun Rev. 2013;12(10):976-89. 10. Pludowski P, Karczmarewicz E, Bayer M, Carter G, Chlebna-Sokó¿ D, Czech-Kowalska J, D¿bski R, Decsi T, Dobrza¿ska A, Franek E, G¿uszko P, Grant WB, Holick MF, Yankovskaya L, Konstantynowicz J, Ksi¿¿yk JB, Ksi¿¿opolska-Or¿owska K, Lewi¿ski A, Litwin M, Lohner S, Lorenc RS, Lukaszkiewicz J, Marcinowska-Suchowierska E, Milewicz A, Misiorowski W, Nowicki M, Povoroznyuk V, Rozentryt P, Rudenka E, Shoenfeld Y, Socha P, Solnica B, Szalecki M, Ta¿a¿aj M, Varbiro S, Zmijewski MA. Practical guidelines for the supplementation of vitamin D and the treatment of deficits in Central Europe - recommended vitamin D intakes in the general population and groups at risk of vitamin D deficiency. Endokrynol Pol. 2013;64(4):319-27. 11. Hossein-Nezhad A, Holick MF. Vitamin D for Health: A Global Perspective. Mayo Clin Proc. 2013;88(7):720-55. Competing interests I receive funding from Bio-Tech Pharmacal (Fayetteville, AR), the Sunlight Research Forum (Veldhoven), and Medi-Sun Engineering, LLC (Highland Park, IL).