Seasonality in risk of pandemic influenza emergence
Fig 3
The evolving structure of the susceptible population as the flu season unfolds.
For purposes of illustration, we present caricatures of each model through time, assuming that the average degree is 〈k〉 = 6 and that we repeatedly observe the same subset of each population. Orange represents individuals susceptible to infection by the pandemic virus and the contacts between them; gray indicates individuals who are currently or recently infected by the seasonal virus, and thus immune to pandemic infection. The empirical (top) and homogeneous (bottom) networks experience different structural changes in pandemic susceptibility throughout the flu season. In January, prior to the onset of flu season, both networks are fully susceptible. Just following the seasonal epidemic peak (March), both networks are at the base of their refractory period, with many nodes resistant to the pandemic virus. Even with the same number of susceptible nodes, the empirical network is more disrupted than the homogeneous network. Highly connected (hub) nodes are more vulnerable to seasonal infection than less connected nodes and, once removed by immunity, critically disconnect the susceptible portion of network. After the seasonal epidemic has subsided (June), short-term immunity has largely waned in both models, leaving them vulnerable to pandemic invasion.