Not all seasonal influenza viruses circulate around the world in the same way, a study published in Nature this week demonstrates. Compared with A/H3N2, the most frequent and severe influenza subtype, A/H1N1 and B viruses do not show the same degree of global movement but persist for longer periods of time locally. Understanding the dynamics of influenza viruses is an important scientific and public health challenge; the latest research provides new insights on the circulation patterns of A/H1N1 and B viruses, which have not been studied in as much detail as the more common A/H3N2 viruses.
An analysis of nearly 10,000 seasonal human influenza virus sequences over a 12-year period, performed by Colin Russell, Trevor Bedford, and colleagues, reveals that A/H3N2 viruses move between regions more frequently than A/H1N1 and B viruses. One reason for this variation may be the rates at which these viruses evolve, the authors suggest. A/H3N2 viruses evolve relatively quickly and routinely infect people of all ages, but A/H1N1 and B viruses evolve more slowly and mainly infect children. As children travel long distances much less than adults this factor may account for differences in the global circulation of viruses. The slower rates of evolution of A/H1N1 and B viruses compared with A/H3N2 viruses may also explain why A/H1N1 and B viruses cause fewer major epidemics than A/H3N2 viruses.