Open Access Research article

Let the sun shine in: effects of ultraviolet radiation on invasive pneumococcal disease risk in Philadelphia, Pennsylvania

Alexander NJ White12, Victoria Ng15, C Victor Spain4, Caroline C Johnson4, Laura M Kinlin13 and David N Fisman123*

Author Affiliations

1 Child Health Evaluative Sciences, Research Institute of the Hospital for Sick Children, 123 Edward Street, Toronto M4V 1X6, Canada

2 Faculty of Medicine, University of Toronto, 1 King's College Circle, Toronto M5S 1A8, Canada

3 Dalla Lana School of Public Health, University of Toronto, Toronto M4T 3M7, Canada

4 Division of Disease Control, Philadelphia Department of Public Health, 500 South Broad Street, Philadelphia 19146, USA

5 National Centre for Epidemiology and Population Health, The Australian National University, Canberra 0200, Australia

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BMC Infectious Diseases 2009, 9:196  doi:10.1186/1471-2334-9-196

Published: 4 December 2009



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.


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.


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).


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 temperature.