Introduction: Demographic changes as a result of evacuation in the acute phase of the 2011 Fukushima nuclear disaster are not well evaluated. We estimated post-disaster demographic transitions in Minamisoma City - located 14-38 km north of the nuclear plant - in the first month of the disaster; and identified demographic factors associated with the population remaining in the affected areas. Materials and methods: We extracted data from the evacuation behavior survey administered to participants in the city between July 11, 2011 and April 30, 2013. Using mathematical models, we estimated the total population in the city after the disaster according to sex, age group, and administrative divisions of the city. To investigate factors associated with the population remaining in place after the disaster, a probit regression model was employed, taking into account sex, age, pre-disaster dwelling area, and household composition. Results: The overall population decline in Minamisoma City peaked 11 days after the disaster, when the population reached 7,107 people - 11% of the pre-disaster level. The remaining population levels differed by area: 1.1% for mandatory evacuation zone, 12.5% for indoor sheltering zone, and 12.6% for other areas of the city. Based on multiple regression analyses, higher odds for remaining in place were observed among men (odds ratio 1.72 [95% confidence intervals 1.64-1.85]) than women; among people aged 40-64 years (1.40 [1.24-1.58]) than those aged 75 years or older; and among those living with the elderly, aged 70 years or older (1.18 [1.09-1.27]) or those living alone (1.71 [1.50-1.94]) than among those who were not. Discussion: Despite the evacuation order, some residents of mandatory evacuation zones remained in place, signaling the need for preparation to respond to their post-disaster needs. Indoor sheltering instructions may have accelerated voluntary evacuation, and this demonstrates the need for preventing potentially disorganized evacuation in future nuclear events.
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