Survival analysis of the duration of time to the first confirmed case in each county indicated that all 4 factors related to domestic travel or human mobility were significantly associated with the invasion of pandemic influenza in the Cox univariate analysis (Table 1) . Population density and the density of medical facilities also showed a significant association. The significant effect of being intersected by railways disappeared, and the density of medical facilities showed borderline significance after correction for other factors in multivariate analysis, whereas being intersected by national highways and freeways and proximity to airports and higher population den-sity remained as significant factors, all showing a positive association (Table 1) . Table 2 shows that all climatic factors (except precipitation), school summer vacation and public holidays, proportion of the school-age population, population density, and the density of medical facilities were significantly associated with the extent of local transmission in univariate multilevel Poisson regression. School summer vacation and public holidays showed a significant negative association with the incidence of pandemic influenza. The significant effect of the proportion of schoolage children disappeared after correction for other factors; thus, temperature, relative humidity, school summer vacation and public holidays, population density, and the density of medical facilities remained as significant factors in multivariate analysis. Temperature showed a peak pattern, with the highest incidences for the range from 0°C to 10°C, which was also reflected in the statistically significant quadratic term.