We studied approximately 14,000 visits during the discrete year and 32,000 visits during concomitant years. Viral results were unavailable for approximately 75% of respiratory visits and multiple imputation was used to impute values. During the discrete year, sensitivity and specificity were 0.49 (95% Confidence Interval [CI]: 0.30, 0.68) and 0.72 (CI: 0.70, 0.74) respectively, for the ''Febrile'' definition and 0.29 (CI: 0.13, 0.54) and 0.89 (CI: 0.87, 0.90) for the ''ILI'' definition. ILI sensitivity was 2.05 (CI: 1.08, 3.91) times greater and its false positive fraction 44% (CI: 37%, 49%) lower in concomitant years compared to the discrete year. Greater sensitivity and false positive fractions (1-specificity) tended to be produced by the febrile definitions than by the ILI definitions; and by definitions derived from CC or diagnoses as compared to those from CC alone. The false positive fraction of all syndrome definitions was higher in younger children compared to older children.