Accepting all of these shortcomings, some of which are inevitable for such phylogenetic studies (since not all samples can be collected and sequenced from all infected individuals from all over the world for any particular virus), there are still some useful conclusions that can be gained from this study: i) from Figures S1 and S2, there is at least one example of a virus that reappears, relatively unchanged between consecutive influenza seasons, and which can be seen as some evidence to support the 'latency' hypothesis [19] . Since the same viruses show this same pattern of clustering in both their HA and NA genes, this reduces the likelihood that this was due to a reassortment event in one of these genes, i.e. it is less likely for the same viruses to have reassorted both the HA and NA genes during the same years -though of course this possibility cannot be ruled out. Whole genome sequencing would be useful to confirm this if resources are available in the future. ii) from Figures S3 and S4 , it is difficult to say whether viruses from Hong Kong preceded (or gave rise to) those from elsewhere, since viruses from outside Hong Kong can be found to both pre-date and post-date those isolated from Hong Kong. However, there are several examples of similar viruses being isolated in different parts of the world at about the same time (as shown in some of the red ellipses in Figures S3 and S4 Figures S3 and S4) , though this may not necessarily mean that the Fuj/411/02-like viruses originated from there.