Response

We read with great interest the study by Rong et al,^{Reference Rong, Wang, Cheng, Li and Zhao1} which analysed global trends in the burden of depressive disorders – stratified by gender, age and the Sociodemographic Index (SDI) – at global, regional and national levels. The study found a general decline in the global burden of depressive disorders from 1990 to 2021, as reflected in trends in incidence, prevalence and disability-adjusted life years (DALYs) across five SDI quintiles. This decline aligns with findings from other studies, which also highlight the atypical patterns in depressive disorder burden across different SDI regions.^{Reference Wu, Fan, Xia, Zhou, Wang and Feng2} However, it is important to note that this burden continues to increase in some regions and countries.^{Reference Liu, He, Yang, Feng, Zhao and Lyu3} All the above-mentioned findings underscore the urgent need to quantify health inequalities to guide the development of targeted and effective health strategies in the future.

Income is a key social determinant of population health, playing a significant role in the allocation of healthcare resources. The SDI – a composite measure incorporating per capita income, average years of education and fertility rates – serves as an effective tool for assessing socioeconomic progress. In our study, we analysed incidence, prevalence and DALYs data for 204 countries and territories from the Global Burden of Disease (GBD) 2021 database. To evaluate inequalities in the burden of depressive disorders, we utilised two standard measures: the Slope Index of Inequality (SII) and the Concentration Index, which represent absolute and relative inequality, respectively. The SII, derived from regression of national and regional DALY rates against cumulative SDI rankings, quantifies disparities across income levels, providing a comprehensive assessment of inequality trends. Meanwhile, the Concentration Index is derived from the Lorenz curve, which represents the cumulative distribution of incidence, prevalence and DALYs relative to SDI rankings.^{Reference Peng, Xu and Tang4} Both the SII and Concentration Index indicate that negative values correspond to the association of higher SDI levels with lower age-standardised incidence, prevalence and DALY rates.

From 1990 to 2021, the SII for depression incidence – reflecting absolute inequality – declined globally from −1507.79 (95% CI: −2068.38 to −947.19) to −971.89 (95% CI: −1630.82 to −312.96) (Fig. 1a). A similar decreasing trend was observed for depression prevalence, with SII declining from −1239.28 (95% CI: −1623.71 to −854.86) to −876.99 (95% CI: −1322.51 to −431.48) per 100 000 population (Fig. 1c). Likewise, the SII for DALYs due to depressive disorders decreased from −202.55 (95% CI: −278.92 to −126.19) in 1990 to −135.55 (95% CI: −230.47 to −40.63) in 2021 (Fig. 1e). These declines indicate a gradual narrowing of absolute socioeconomic disparities in the burden of depressive disorders between high- and low-SDI countries. However, during the same period, relative inequality − as measured by the Concentration Index − slightly increased. For example, the Concentration Index for incidence changed marginally from −0.067 (95% CI: −0.092 to −0.043) to −0.069 (95% CI: −0.096 to −0.042), while for prevalence it remained relatively stable, shifting from −0.0544 (95% CI: −0.0708 to −0.0379) in 2019 to −0.0544 (95% CI: −0.0730 to −0.0357) in 2021 (Fig. 1b, d). Meanwhile, the Concentration Index for DALYs increased from −0.056 (95% CI: −0.076 to −0.036) in 1990 to −0.058 (95% CI: −0.081 to −0.035) in 2021 (Fig. 1f). These findings suggest that, although the absolute burden gap has narrowed, relative inequality persists and may even be widening slightly.

Fig. 1

Absolute and relative cross-country inequality in ASIR, ASPR and ASDR, 1990–2021. (a) health inequality regression curves for ASIR of depressive disorders; (b) concentration curves for ASIR of depressive disorders; (c) health inequality regression curves for ASPR of depressive disorders; (d) concentration curves for ASPR of depressive disorders; (e) health inequality regression curves for ASDR of depressive disorders; (f) concentration curves for ASDR of depressive disorders. ASIR, age-standardised rates for incidence; ASPR, age-standardised rates for prevalence; ASDR, age-standardised DALY rate; DALY, disability-adjusted life year; SDI, Sociodemographic Index; Pop, population.

While global trends point to an easing of absolute inequality, regional disparities have followed divergent trajectories. In high-income North America, for example, inequality worsened across all three indicators. The SII for DALYs increased from −694.68 (95% CI: −1356.03 to −33.34) in 1990 to −779.94 (95% CI: −1029.94 to −529.95) in 2021, indicating a widening of the absolute gap. Correspondingly, the Concentration Index for DALYs rose from −0.010 (95% CI: −0.017 to −0.004) to −0.027 (95% CI: −0.028 to −0.025) over the same period (Supplementary Fig. 1c). In contrast, East Asia showed signs of declining inequality in incidence, prevalence and DALYs. Both the SII and the Concentration Index decreased in absolute value, suggesting a possible narrowing of socioeconomic disparities (Supplementary Fig. 1). However, these changes were not statistically significant because their confidence intervals crossed zero. Nonetheless, the direction of change may still reflect a positive shift towards greater regional equity.

From a global perspective, the burden of depressive disorder is gradually decreasing. However, regional disparities persist, with some areas experiencing different trends in disease burden. These regional differences are shaped by a complex interplay of socioeconomic, cultural and environmental factors. For example, high-income North America faces challenges such as the high levels of economic development, education and social pressure. Additionally, environmental factors such as natural disasters and climate change have a profound impact on mental health, contributing to a higher burden of depressive disorder in certain regions.^{Reference Obradovich, Migliorini, Paulus and Rahwan5} These findings underscore the urgent need for region-specific interventions that address the unique challenges of each area. A one-size-fits-all approach is insufficient; instead, targeted strategies are essential to effectively reduce the disease burden and promote equitable mental health outcomes globally.