In this particular study, there is also a problem of sample bias as these sequences were obtained from only hospitalized children (rather than from those who remained in the community), and may therefore reflect the influenza virus population isolated from the more clinically severely ill patients (or those with more concerned, anxious parents). However, there was a rationale for deliberately selecting children's samples for this study. The reason for this is that, especially in Hong Kong, unlike adults, children of school age (1-10 years old) are more likely to stay close to home and not travel far from home, which would minimize any importation of influenza viruses from overseas. This would therefore reduce this confounding factor when assessing the migration vs latency hypotheses as an explanation for the underlying mechanism of influenza seasonality in Hong Kong.