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

ABSTRACT: BACKGROUND: Environmental factors have been associated with the 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.