Sociodemographic patterns of COVID-19 mortality: the 2020 Japanese census-linked mortality database.

Background: Owing to the lack of sociodemographic mortality statistics in Japan, linking existing individual statistical data could be cost-effective and, therefore, highly sustainable for monitoring health inequalities regarding urgent health issues. We investigated nationwide coronavirus disease 2019 (COVID-19) cause-specific mortality by sociodemographic characteristics compared with all-cause mortality in Japan, using our unique linkage method among non-institutionalised citizens. Methods: Using the 2020 Japanese census-linked mortality database, we calculated age-standardised mortality rates from all-cause and COVID-19 by region, marital status, household size ('living alone' to 'five or more'), educational level, area deprivation index (ADI: municipality-level population quantiles), and occupational class. We then applied multivariable modified Poisson regression analysis to investigate the relationship between all-cause and COVID-19 mortality with sociodemographic characteristics, excluding occupational class. All analyses were performed by sex and period (i.e., during Delta variant dominance in 2021 and Omicron variant dominance in 2022). ADI analyses also accounted for metropolitan residency (the Tokyo and Osaka metropolitan areas vs. non-metropolitan areas). Findings: This analysis included 80,135,688 non-institutionalised Japanese individuals (aged 30-89 years) linked to 1,895,080 all-cause deaths between October 2020 and December 2022, including 34,213 COVID-19-related deaths. After controlling for sociodemographic characteristics, marital status (e.g., for single men, mortality rate ratio: 2.02 (95% confidence intervals [CI]: 1.90-2.14), compared with married men), and low education level (e.g., low-educated women: 1.49 [95% CI: 1.38-1.61], compared with high-educated women) were associated with increased COVID-19 mortality, similar to the trends in all-cause deaths. Additionally, having a large household was associated with increased COVID-19 mortality rate ratio (e.g., women living with five or more people: 1.69 [95% CI: 1.54-1.84], compared with women living alone; p for trend < 0.0001: using ordinal variables of household size). These patterns were consistent across sexes and variant periods. COVID-19 mortality was not significantly associated with ADI (mortality rate ratio for men in the most deprived municipalities compared with in the least deprived municipalities: 0.95 [95% CI: 0.89-1.01], p for trend = 0.069; women: 0.97 [95% CI: 0.90-1.06], p for trend = 0.285), contrasting with all-cause mortality rate ratios, which increased with deprivation. ADI-related trends varied by variant period and metropolitan status. Interpretation: In Japan, sociodemographic patterns of COVID-19 mortality for the non-institutionalised population related to marital status and education reflect underlying health inequalities-all-cause mortality patterns. However, the lack of an association with area deprivation marks a divergence from typical all-cause mortality patterns. Notably, residing in large households was uniquely associated with increased COVID-19 mortality, an uncommon feature among cause-specific mortality patterns. Funding: Japan Society for the Promotion of Science KAKENHI (grant number: 23K16341).