The recent paper on 2009 pandemic influenza A (H1N1) on the island of La Réunion reported low clinical burden during pregnancy [1]. This is in contrast to findings elsewhere, where pregnant women often have increased risk of complications associated with influenza as mentioned in [1]. The authors stated “The reasons for which the clinical burden of H1N1/09 influenza virus may differ worldwide raise questions about a differential local viral-strain effect and public health preparedness, notably in timely access to special care and antiviral treatments.” In this comment, I ague that pregnant women in La Réunion have higher serum 25-hydroxyvitamin D [25(OH)D] levels than pregnant women in higher latitude locations.

Influenza is largely seasonal due to annual variations in solar ultraviolet-B (UVB) doses and vitamin D production [2,3]. Low absolute humidity in winter also plays a role [4]. Two randomized controlled trials found that higher oral intake of vitamin D reduced the risk of influenza [5,6]. Higher serum 25(OH)D levels also reduce the risk of complications of influenza such as pneumonia [7].

Recent research suggests that pregnant women require more vitamin D than other people [8-10]. Pregnant women in La Réunion very likely have higher serum 25(OH)D levels than at higher latitude locations due to two reasons: low latitude and a largely agricultural economy. To investigate whether higher serum 25(OH)D levels largely explain lower clinical burden of pandemic (or seasonal) influenza A in La Réunion, serum 25(OH)D levels of pregnant women could be measured and compared to levels elsewhere, and levels of infected pregnant women could be compared to uninfected pregnant women. In addition, when pregnant women or others develop influenza A, they could be given large doses of vitamin D, e.g., 50,000 IU, and followed to see whether outcomes were better.

William B. Grant, Ph.D.
Sunlight, Nutrition, and Health Research Center (SUNARC)
P.O. Box 641603
San Francisco, CA 94164-1603, USA
www.sunarc.org, wbgrant@infionline.net

References
1. Gérardin P, El Amrani R, Cyrille B, Gabrièle M, Guillermin P, et al. (2010) Low clinical burden of 2009 pandemic influenza A (H1N1) infection during pregnancy on the island of La Réunion. PLoS One 5:e10896.
2. Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, et al. (2006) Epidemic influenza and vitamin D. Epidemiol Infect 134:1129-40.
3. Cannell JJ, Zasloff M, Garland CF, Scragg R, Giovannucci E. (2008) On the epidemiology of influenza. Virol J 5:29.
4. Shaman J, Pitzer VE, Viboud C, Grenfell BT, Lipsitch M. (2010) Absolute humidity and the seasonal onset of influenza in the continental United States. PLoS Biol 8:e1000316.
5. Aloia JF, Li-Ng M. (2007) Re: epidemic influenza and vitamin D. Epidemiol Infect 135:1095-6; author reply 1097-8.
6. Urashima M, Segawa T, Okazaki M, Kurihara M, Wada Y, Ida H. (2010) Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr 91:1255-60.
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7. Grant WB, Giovannucci D. (2009) 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 1: 215-9.
8. Hollis BW. (2007) Vitamin D requirement during pregnancy and lactation. J Bone Miner Res 22 Suppl 2:V39-44.
9. Merewood A, Mehta SD, Chen TC, Bauchner H, Holick MF. (2009) Association between vitamin D deficiency and primary cesarean section. J Clin Endocrinol Metab 94:940-5.
10. Lewis S, Lucas RM, Halliday J, Ponsonby AL. (2010) Vitamin D deficiency and pregnancy: From preconception to birth. Mol Nutr Food Res May 3. [Epub ahead of print]