Does Travel Spread Infection?—Effects of Social Stirring Simulated on SEIRS Circuit Grid

Previous models of the spread of viral infection could not explain the potential risk of non-infectious travelers and exceptional events, such as the reduction in infected cases with an increase in travelers. In this study, we provide an explanation for improving the model by considering two factors. First, we consider the travel of susceptible (S), exposed (E), and recovered (R) individuals who may become infected and infect others in the destination region in the near future, as well as infectious (I). Second, people living in a region and those moving from other regions are treated as separate but interacting groups to consider the potential influence of movement before infection. We show the results of the simulation of infection spread in a country where individuals travel across regions and the government chooses regions to vaccinate with priority. As a result, vaccinating people in regions with larger populations better suppresses the spread of infection, which turns out to be a part of a general law that the same quantity of vaccines can work efficiently by maximizing the conditional entropy Hc of the distribution of vaccines to regions. This strategy outperformed vaccination in regions with a larger effective regeneration number. These results, understandable through the new concept of social stirring, correspond to the fact that travel activities across regional borders may even suppress the spread of vaccination if processed at a sufficiently high pace. This effect can be further reinforced if vaccines are equally distributed to local regions.