A new model to simulate the progression of an influenza pandemic at the national level has been developed by Japanese researchers. Inspired by work done for the Tokyo metropolitan area, this model takes into account a phenomenon particularly relevant in the case of the spread of infectious disease in Japan, but neglected by previous studies: the possibility of contamination during travel daily transportation between home and workplace. An influenza pandemic occurs when a new variant of the virus against which the world population has no immunity appear. This happened 3 times in the last 100 years: in 1918 with the Spanish

flu, especially lethal (40 million dead), in 1957 with the Asian flu and 1968 with the Hong Kong Flu. If no one knows when the next pandemic emerges, the recent cases of transmission of H5N1 to humans induce the WHO and governments around the world to prepare as much as possible for a pandemic devastating to health and the global economy. In this context, it is important to understand how the virus can spread in order to implement effective measures to contain it. Covering 9 regions and 72 million Japanese, or more than half of the population, the model uses the data from individual travel, information collected from random individuals about their travel, means of transport that they use and the different places they occupy during the day. A number of parameters, as the period of contagion of an infected individual, the rate of transmission of the disease or the likelihood of displacement in each of 9 regions, based on the results of previous studies. Starting from a single individual back in Tokyo two days after being infected during a trip, the model predicts about 360,000 cases at the end of two weeks throughout the country, mostly located in metropolitan Tokyo and the Kansai. Even with optimistic assumptions about the early identification of the virus (6 days after initial infection and two after the onset of symptoms), it seems difficult to develop effective quarantine in these two regions. This type of measure could be useful against smaller cities, where the model predicts a slower spread. As reported by the researchers, the study presents a number of limitations that they wish to correct in the next simulations. These limitations are mainly related to a lack of computing power: thus, only the first phase of the disease was simulated, the influence of countermeasures against - school closures, vaccinations .Could not be taken into account and a single iteration of the simulation was conducted. Finally, several regions where there are no data on individual travel were excluded. However, the purpose of the study is to give a first idea of realism achieved by the model, to promote its use to policymakers.