Although travel restrictions and social measures during the COVID-19 pandemic led to a dramatic global drop in seasonal influenza cases, certain influenza lineages in specific regions kept the virus circulating and evolving, according to a new study. This was true in tropical areas with fewer travel restrictions, for example, including South and West Asia. The spread of seasonal influenza is closely tied to social behavior, particularly air travel, and to the periodic evolution of new virus strains that evade immunity from prior infections or vaccinations.
In 2020, nonpharmaceutical interventions (NPIs) introduced to combat COVID-19 – such as lockdown policies, mandated social distancing, masking, and travel bans – dramatically impacted influenza virus transmission and evolution. Due to these interventions, cases of seasonal influenza caused by A subtypes H1N1 and H3N2, as well as influenza B subtypes Victoria and Yamagata, declined sharply across the globe. Here, Zhiyuan Chen and colleagues investigated how these changes affected the spread, distribution, and evolutionary dynamics of seasonal influenza lineages.
Using a phylodynamic approach, Chen et al. combined epidemiological, genetic, and international travel data from before, during, and after the COVID-19 pandemic and found that the onset of the pandemic led to a shift in the intensity and structure of international influenza transmission. Although influenza cases significantly dropped globally during the pandemic'.
