BACKGROUND: Concerns have been raised that the COVID-19 pandemic could increase risk for adverse mental health outcomes, especially in young adults, a vulnerable age group. We investigated changes in depression and anxiety symptoms (overall and severe) from before to during the pandemic, as well as whether these changes are linked to COVID-19-related stressors and pre-existing vulnerabilities in young adults followed in the context of a population-based cohort. METHOD: Participants (n = 1039) from the Quebec Longitudinal Study of Child Development reported on their depression (Centre for Epidemiological Studies Depression Scale, short form) and anxiety (General Anxiety Disorder-7 Scale) symptoms and completed a COVID-19 questionnaire during the first wave of the COVID-19 pandemic in the summer of 2020 (age 22 years). Assessments at age 20 (2018) were used to estimate pre-pandemic depression and anxiety symptom severity. RESULTS: While mean levels of depression and anxiety symptoms did not change from before to during the first wave of the COVID-19 pandemic (e.g., the mean of depressive symptoms was 9.30 in 2018 and 9.59 in 2020), we observed a slight increase in rates of severe depression (scores ≥ 21) from before (6.1%) to during (8.2%) the pandemic. Most COVID-19-related variables (e.g., loss of education/occupation, frequent news-seeking) - except living alone - and most pre-existing vulnerabilities (e.g., low SES, low social support) were not associated with changes in depression or anxiety symptoms. However, results varied as a function of pre-pandemic levels of depression and anxiety: depression and anxiety symptoms increased among adults with the lowest levels of symptoms before the pandemic, while they decreased among those with the highest levels of symptoms, possibly reflecting a regression to the mean. CONCLUSIONS: Depression and anxiety symptoms in young adults from Québec in Summer 2020 were mostly comparable to symptoms reported in 2018. Most COVID-19-related stressors and pre-existing vulnerabilities were not associated with changes in symptoms, except living alone and pre-existing symptoms of depression and anxiety. However, the increased rate of severe depression warrants further investigation.< Réduire