Our previous cohort study used national data, and found a relationship between non-permanent employment and mental health deterioration during the COVID-19 induced state of emergency in Japan. However, non-response (approximately 22%) was a problem. Importantly, this non-response bias may limit the interpretability of our survey results. This study aimed to reveal the factors associated with non-response utilizing a follow-up survey based on our baseline data. We conducted a web-based survey of the working-age population in Japan carried once from March 26 to April 6, 2020, and again from June 26 to July 2, 2020. From a cohort of 3,001 Japanese workers, 650 and 2,351 did and did not drop out from the follow-up survey, respectively. We find that non-respondents had higher Center for Epidemiological Studies Depression Scale (CES-D) (non-respondents versus respondents: 18 [12-27] versus 15 [11-25], p < 0.001), lower sense of coherence (SOC) (non-respondents versus respondents: 51 [45-56] versus 52 [47-58], p < 0.001), and lower health-related quality of life (HRQOL) (non-respondents versus respondents: 0.867 [0.799-1.000] versus 0.895 [0.823-1.000], p < 0.001) scores. Logistic regression analyses revealed that the risk factors associated with causing non-response were younger age (odds ratio [OR] = 8.358), low level income (OR = 1.997), being single (OR = 1.435), not being head of household (OR = 1.377), and low SOC (OR = 1.310). Higher HRQOL scores were inversely associated with non-response (OR = 0.453). Thus, our findings raise the concern that non-response bias may underestimate mental health deterioration in non-permanent employees during our follow-up surveys. A repeated cross-sectional study may complement the assessment of prevalence of adverse mental health symptoms in Japan at a given time point during the COVID-19 pandemic.
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