Negative binomial regression analysis. Results from negative binomial regression analysis suggest influenza A and B (combined) activity is associated with IPD (incidence rate ratio = 1.09 case of IPD per 100 influenza cases, p,0.001), after adjustment for seasonal oscillation, multi-year trends, humidity, temperature, and UV index. Furthermore, there is an inverse association between average clear-sky UV index, both during the prior week, and 3 wk prior, and IPD (Table 4 ). Increased relative humidity was also associated with reduced risk of case occurrence. Models constructed using influenza A counts rather than total counts were identical in covariates and model performance to those constructed using total influenza counts (consistent with the large predominance of influenza A infections identified by the FluWatch system). We identified no significant interactions between influenza counts, UV index, and humidity. There was no significant association between influenza B incidence and IPD risk in either univariable or multivariable models. Model results were robust when we excluded influenza seasons that appeared to be outliers with respect to incidence (2003-2004 and 2008-2009 ) (unpublished data).