Blame it on Commuters: U.S. Transportation Networks and their Influence on the Genetic Structure of Influenza
It is well established that the flu virus spreads through human contact, but for the first time, Emory researchers have analyzed U.S. commuter networks and have observed a dramatic correlation between strong interstate commuter links and genetic similarity of the influenza strains observed in those regions. Researchers mapped out both aviation and commuter transportation networks across the United States, assigning each state as a specific node within the network map.
It was found that the commuter network, shown below, was highly organized, with connections often occurring across state borders. The volume of commuters traveling from one node to another was indicated by the color of this edge (red indicating the highest volume of commuters). In this way, edges with a higher number of commuters (such as red edges) can be seen as strong ties within the network, while those edges with a lower volume of commuters (such as blue and green edges) can be seen as weak ties within the network. Additionally, this image shows that connections in the east were often stronger than in the west.
Using this information, the researchers compared the aviation network, commuter network, and geographic distance with the genetic distance of influenza strains H1N1 and H3N2. In their results, they found that the correlation between genetic distance and commuter network connectivity was significantly greater than aviation network connectivity and geographic distance. More specifically, the research showed a higher level of genetic similarity of H1N1 substrains among commuter network nodes with stronger ties (i.e. a higher volume of commuters). The genetic similarity of flu substrains among nodes with stronger (red) ties showed that the strength of commuter network ties played a significant role in flu transmission.
With respect to the network behavior in this commuter network map, strong tie strength among select node triads was a noticeable characteristic. For example, among the three nodes NJ, NY, and PA, we see that red-colored strong ties connect each of the nodes together, satisfying the Strong Triadic Closure principle; in these three states, genetic similarity of flu strains was also found to be higher. Other triads, such as DC, MD, and VA, demonstrate similar behavior with respect to commuter connectivity as well as genetic similarity of flu strains. Overall, this research presents an intriguing connection between graph theory and influenza research as it highlights the importance of commuter network behavior, the strength of ties between nodes, and the impact of transportation networks on the spread of flu strains in different regions.
Source:
The Role of Human Transportation Networks in Mediating the Genetic Structure of Seasonal Influenza in the United States
http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1004898