The second comparison we performed was between the sentinel clinical and the non-sentinel virological data. This was an important validity check because two independent surveillance systems were compared. Many countries (e.g. the U.S.) do not collect sentinel virological data, but base the virological assessment of influenza activity solely on data from non-sentinel sources [15] . Our study found a good match between sentinel clinical and nonsentinel virological sources. Again, the clinical rates can sometimes be unreliable during seasons when activity is very low, which may lead to mismatches. On the other hand, a large outbreak in a major hospital or region may be another important factor that can affect the non-sentinel data. Such localized outbreaks may lead to increased non-sentinel reports that are not picked up by the sentinel clinical reports, resulting in a mismatch. Taken together, we do favour the sentinel collection of virological data because the approach is more systematic, less prone to pre-selection and differs less among countries than clinical sampling.