Significant interaction between temperature and influenza on the mortality risks was only found in cardiorespiratory mortality in Hong Kong (p < 0.05), and the interaction between relative humidity and influenza was found significant for all-cause mortality in both Guangzhou and Hong Kong, and for cardiorespiratory mortality only in Guangzhou ( Table 3 ). The patterns of influenza impact across the low to high temperature periods were not consistent among the two cities. In Guangzhou, the highest risk of mortality tended to be observed in the middle-temperature period, whereas for Hong Kong the largest changes in risk were found in the periods with high temperature for the three mortality categories ( Table 3 ). An increasing pattern of influenza associated mortality risks could be observed along low-, mid-and high-relative humidity periods in Guangzhou and Hong Kong, but most of estimates for low-and high-periods were not statistically significant ( Table 3) .