American Journal of Epidemiology Advance Access originally published online on March 16, 2007
American Journal of Epidemiology 2007 165(12):1434-1442; doi:10.1093/aje/kwm012
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ORIGINAL CONTRIBUTIONS |
Seasonality of Influenza in Brazil: A Traveling Wave from the Amazon to the Subtropics
1 Fogarty International Center, National Institutes of Health, Bethesda, MD
2 National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD
3 Mechanical Engineering Department, Universidade Federal de Santa Catarina, Florianópolis, Brazil
4 Secretariat of Health Surveillance, Ministry of Health, Brasília, Brazil
Reprint requests to Dr. Wladimir J. Alonso, National Institutes of Health, Fogarty International Center, Bethesda, MD 20892-6710 (e-mail: alonsow{at}mail.nih.gov).
Received for publication July 14, 2006. Accepted for publication November 7, 2006.
Influenza circulation and mortality impact in tropical areas have not been well characterized. The authors studied the seasonality of influenza throughout Brazil, a geographically diverse country, by modeling influenza-related mortality and laboratory surveillance data. Monthly time series of pneumonia and influenza mortality were obtained from 1979 to 2001 for each of the 27 Brazilian states. Detrended time series were analyzed by Fourier decomposition to describe the amplitude and timing of annual and semiannual epidemic cycles, and the resulting seasonal parameters were compared across latitudes, ranging from the equator (+5°N) to the subtropics (–35°S). Seasonality in mortality was most pronounced in southern states (winter epidemics, June–July), gradually attenuated toward central states (15°S) (p < 0.001), and remained low near the equator. A seasonal southward traveling wave of influenza was identified across Brazil, originating from equatorial and low-population regions in March–April and moving toward temperate and highly populous regions over a 3-month period. Laboratory surveillance data from recent years provided independent confirmation that mortality peaks coincided with influenza virus activity. The direction of the traveling wave suggests that environmental forces (temperature, humidity) play a more important role than population factors (density, travel) in driving the timing of influenza epidemics across Brazil.
Brazil; climate; geographic locations; influenza, human; mortality; pneumonia; seasons
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