Two-step criteria to define (likely) causes for LRI hospitalization clusters detected in 1999–2006.
* As evaluated by the right-sided Fisher's exact test for 2×2 Tables (alpha≤0.01) of hospitalizations within vs hospitalizations outside of the cluster-signal. The proportion of hospitalizations with a specific characteristic (e.g. legionnaires' disease as discharge diagnoses, or age 20–49 yrs) can be significantly higher among hospitalizations within the cluster-signal than the proportion outside of the cluster-signal. ** For the ILI-cluster-signals we could only use 4 major regions as spatial resolution. Overlap in time between LRI and ILI-cluster-signals was defined as occurrence of weekly ILI-cluster-signals within 2 weeks (+/−) around LRI-cluster-signals. ***The annual influenza season was defined as all weeks with a national weekly ILI-incidence ≥3 per 10.000 pop. **** Possibly unreported/undetected local LRI-outbreaks by undetected pathogens.