In summary, this study has provided additional data from Hong Kong to support a mainly migratory mechanism to explain the underlying seasonality of influenza A(H3N2) viruses. However, there may be small localities of so-called 'latency' where viruses remain circulating at low levels within that local population, to reemerge during the influenza season of the following year, with relatively little genetic change. This may affect only a minority of these populations, with the majority being infected by newly imported influenza viruses from elsewhere. These concepts are summarized in Figure 1 .