Case-crossover analysis. We used a case-crossover approach to evaluate short-term associations between influenza A and B (combined), influenza A, and influenza B infection and IPD occurrence during influenza seasons in order to provide a means for evaluating the association between brief exposures and rare outcomes. The design is characterized by ''self matching,'' in that cases serve as their own controls. A ''case'' thereby is a day on which a case occurred, whereas a ''control'' is an adequately selected day on which a case did not occur [21, 22] . A time-stratified 2:1 matched case-crossover design in which hazard periods were defined as the reported date of IPD onset from the Toronto Invasive Bacterial Diseases Network database was incorporated. Beginning on January 1, 1995, the person-time at risk was divided into 3-wk time strata. Control periods were chosen by matching the hazard period by day of the week within the stratum, and could both precede, both follow, or straddle the hazard period [23, 24] . Random directionality of control selection was used in order to avoid biases occurring with unidirectional or uniform bidirectional control selection [24] . We evaluated the effects of influenza exposures and environmental covariates through construction of conditional logistic regression models with time lags from 0 to 4 wk.