Solar ultraviolet-B (UVB) irradiance reduces the risk of community-acquired pneumonia through production of vitamin D. An observational study in Philadelphia found that pneumonia was most common in winter when solar UVB doses were lowest; the effect was more pronounced among younger people:
Background: Streptococcus pneumoniae is a common cause of community acquired pneumonia and bacteremia. Excess wintertime mortality related to pneumonia has been noted for over a century, but the seasonality of invasive pneumococcal disease (IPD) has been described relatively recently and is poorly understood. Improved understanding of environmental influence on disease seasonality has taken on new urgency due to global climate change.
Methods: We evaluated 602 cases of IPD reported in Philadelphia County, Pennsylvania, from 2002 to 2007. Poisson regression models incorporating seasonal smoothers were used to identify associations between weekly weather patterns and case counts. Associations between acute (day-to-day) environmental fluctuations and IPD occurrence were evaluated using a case-crossover approach. Effect modification across age and sex strata was explored, and meta-regression models were created using stratum-specific estimates for effect.
Results: IPD incidence was greatest in the wintertime, and spectral decomposition revealed a peak at 51.0 weeks, consistent with annual periodicity. After adjustment for seasonality, yearly increases in reporting, and temperature, weekly incidence was found to be associated with clear-sky UV index (IRR per unit increase in index: 0.70 [95% CI 0.54-0.91]). The effect of UV index was highest among young strata and decreased with age. At shorter time scales, only an association with increases in ambient sulphur oxides was linked to disease risk (OR for highest tertile of exposure 0.75, 95% CI 0.60 to 0.93).
Conclusion: We confirmed the wintertime predominance of IPD in a major urban center. The major predictor of IPD in Philadelphia is extended periods of low UV radiation, which may explain observed wintertime seasonality. The mechanism of action of diminished light exposure on disease occurrence may be due to direct effects on pathogen survival or host immune function via altered 1,25-(OH)2-vitamin-D metabolism. These findings may suggest less diminution in future IPD risk with climate change than would be expected if wintertime seasonality was driven by temperature3.
Another example is an ecological study of case-fatality rates during the type A H1N1 influenza pandemic of 1918-19 in the United States, in which the usual cause of death was pneumonia4. The Public Health Service conducted door-to-door surveys of households in 12 communities in the winter of 1918-19 asking whether any members of the household developed influenza and if, so did they die from it. Thus, this was a comprehensive study of case-fatality rates from the pandemic. Case-fatality rates were much higher in northern communities than in southern ones. In an ecological study, it was found that either summertime or wintertime solar UVB indices explained about half of the variance of the case-fatality rates5.
Page last edited: 06 May 2011
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