Background
Data collected during the COVID-19 pandemic suggest an increase in major depressive disorder (MDD) among younger adults. The current study aims to assess the association of age groups and MDD risk before and during the COVID-19 pandemic and quantify the effect of potential mediating variables such as loneliness, social support, resilience, and socioeconomic factors.

Methods
A representative sample of Spanish adults was interviewed before (2019, N = 1880) and during (2020, N = 1103) the COVID-19 pandemic. MDD was assessed using the CIDI, loneliness through the UCLA scale, social support through the OSSS-3, resilience with the 6-BRS, and worsened economic circumstances and unemployment through a single question. Mixed-models were used to study changes in MDD by age group. Regression models were constructed to quantify the association between age and potential mediators, as well as their mediating effect on the association between age group and MDD.

Results
Among the younger age cohorts (18-29 and 30-44 years) the probability of having MDD during the pandemic increased from 0.04 (95 % CI: 0.002-0.09) to 0.25 (0.12-0.39) and from 0.02 (-0.001-0.03) to 0.11 (0.04-0.17), respectively. Some 36.6 % of the association between age and risk of MDD during the pandemic was explained by loneliness (12.0 %), low resilience (10.7 %), and worsened economic situation (13.9 %).

Limitations
Reliance on self-report data and generalizability of the findings limited to the Spanish population.

Conclusions
Strategies to decrease the impact of a pandemic on depressive symptoms among young adults should address loneliness, provide tools to improve resilience, and enjoy improved financial support.