The characteristics of non-permanent employment are likely to have a negative impact on workers' physical and mental health. A growing number of studies worldwide have focused on the health effects of non-permanent employment. However, few studies assessed the association between non-permanent employment and mental health at the beginning of the COVID-19 pandemic in Japan. The present study aimed to clarify any differences in mental health, health- related quality of life (HRQOL), and socioeconomic factors between permanent and non-permanent workers in the early stage of the COVID-19 pandemic in Japan. This study was conducted from March to April 2020. Data for Japanese workers were collected using an anonymous, self-administered, web-based questionnaire. We used the Center for Epidemiological Studies Depression Scale (CES-D) to measure depressive symptoms, the Sense of Coherence (SOC) scale to assess stress-coping ability, and the EQ-5D-5L to assess HRQOL. The overall prevalence of depressive symptoms (CES-D scores ≥16) was 51.5%. Non-permanent workers had higher CES-D scores (permanent vs. nonpermanent workers: 15 [11-25] vs. 17 [12-26], p = 0.012), lower SOC scores (permanent vs. nonpermanent workers: 56 [50–61] vs. 51 [46–57], p < 0.001), and lower HRQOL scores (permanent vs. nonpermanent workers: 1.000 [0.823–1.000] vs. 0.895 [0.823–1.000], p < 0.001). Trend analysis showed an increase in the prevalence of depression symptoms from high-SOC to low-SOC groups, in both permanent and non-permanent whole workers. In low-SOC group of whole workers, median of CES-D score was higher, and HRQOL score was lower significantly. We observed significant negative correlation between SOC and CES-D scores, and significant positive correlation between SOC and HRQOL scores in all SOC groups. The straight-line regression slopes of CES-D and HRQOL versus SOC scores were significantly higher in low-SOC group. Our findings suggest that both mental health and HRQOL could be promoted in Japanese non-permanent workers by strengthening their SOC.
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