Case-crossover results. We were able to identify a significant association between total influenza (A and B) and IPD with a 1-wk lag using a case-crossover approach (odds ratio [95% confidence interval] for one case of IPD per 100 influenza cases, 1.10 [1.02-1.18]) ( Table 5 ). There was no significant association between IPD risk and daily changes in clear-sky UV index, temperature, or relative humidity.