ILI rates from public outpatient clinics and the estimated influenza level from the multivariate model had the highest correlations with laboratory isolation data before the 2009 pandemic (r00.57 and 0.58, respectively) but the former was interrupted during the pandemic period due to activation of the DFC. The estimated influenza level from the multivariate model captured the influenza level well during the pandemic period (r00.76), significantly better than the best surveillance data in the same period (p-value 00.03). The inferred influenza activity index is able to reflect the influenza activity (Fig. 1) .