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Open Access Research article

Effect modification of environmental factors on influenza-associated mortality: a time-series study in two Chinese cities

Lin Yang1, Ping Yan Chen2, Jian Feng He3, King Pan Chan1, Chun Quan Ou2, Ai Ping Deng3, JS Malik Peiris45 and Chit Ming Wong1*

Author Affiliations

1 Department of Community Medicine, School of Public Health, The University of Hong Kong, Hong Kong Special Administrative Region, 5/F William Mong Block, 21 Sassoon Road, Hong Kong, China

2 Department of Biostatistics, School of Public Health and Tropical Medicine, Southern Medical University, Guangzhou, China

3 Guangdong Provincial Centre for Disease Control and Prevention, Guangzhou, China

4 School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region, China

5 HKU Pasteur Research Centre, Pokfulam, Hong Kong Special Administrative Region, China

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BMC Infectious Diseases 2011, 11:342  doi:10.1186/1471-2334-11-342

Published: 14 December 2011

Abstract

Background

Environmental factors have been associated with transmission and survival of influenza viruses but no studies have ever explored the role of environmental factors on severity of influenza infection.

Methods

We applied a Poisson regression model to the mortality data of two Chinese metropolitan cities located within the subtropical zone, to calculate the influenza associated excess mortality risks during the periods with different levels of temperature and humidity.

Results

The results showed that high absolute humidity (measured by vapor pressure) was significantly (p < 0.05) associated with increased risks of all-cause and cardiorespiratory deaths, but not with increased risks of pneumonia and influenza deaths. The association between absolute humidity and mortality risks was found consistent among the two cities. An increasing pattern of influenza associated mortality risks was also found across the strata of low to high relative humidity, but the results were less consistent for temperature.

Conclusions

These findings highlight the need for people with chronic cardiovascular and respiratory diseases to take extra caution against influenza during hot and humid days in the subtropics and tropics.