In our past cohort study, we carried out nationwide web-based surveys to assess mental health deterioration among non-permanent workers over a 1-year period during the COVID-19 pandemic in Japan. The association between non-permanent employment and adverse mental health conditions was found to be the basic characteristic of the original sample. However, high non-response rates and selective drop-out occurred, suggesting that the samples in the follow-up surveys might have been biased. The aim of the present study was to describe changes in sample characteristics among our three surveys and factors associated with non-response in the third survey using data at baseline and the second survey. Our web-based survey of the working-age population in Japan included three waves: March 26 to April 6, 2020; June 26 to July 2, 2020; and October 26 to November 5, 2021. From the original cohort of 3001 Japanese workers, 2351 and 1568 respondents participated in the second and third surveys, respectively. The proportions among respondents of young, single, and non-permanent workers decreased across the three surveys, as did Center for Epidemiological Studies Depression Scale (CES-D) and sense of coherence (SOC) scores; however, EuroQol 5-dimension 5-level (EQ-5D-5L) scores increased across surveys. No differences were found in CES-D, SOC, and EQ-5D-5L scores between non-permanent and permanent workers among respondents in the third survey before and after propensity score matching. The risk factors associated with non-response in the third survey were female sex, younger age at baseline and lower EQ-5D-5L scores in the second survey. The basic characteristics of the sample changed owing to non-response bias, resulting in underestimation of mental health deterioration among non-permanent workers in our cohort study. There is a need for additional cross-sectional studies to complement the assessment of adverse mental health symptoms among workers in Japan at a given time point during the COVID-19 pandemic.
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