Background: To study the longitudinal impact of co-occurring mental health problems, and to identify vulnerable groups in need of mental health support during the COVID-19 pandemic.Methods: Analyses were based on data from 681 French participants in the international COVID-19 Mental Health Study, collected at four times (05/2020-04/2021). Symptoms of depression, anxiety and post-traumatic stress disorder (PTSD) were assessed using the Patient Health Questionnaire 9, the Generalized Anxiety Disorder-7 and the PTSD Check List for DSM-5. We performed k-means for longitudinal data to build trajectories of adults' depression, anxiety and PTSD symptoms and identify subgroups psychologically vulnerable. We then assessed whether mental health trajectories were predicted by lockdown regulations.Results: A high and a low cluster of mental health scores were identified. In both groups, mental health scores varied significantly across time. Levels of all mental health scores were lowest when COVID-19-related restrictions were lifted and highest when restrictions were in place, except for PTSD. No scores returned to the previous level or the initial level of mental health (p < 0.05). Participants with high levels of symptoms were characterized by younger age (OR: 0.98, 95 % CI: 0.97-0.99), prior history of mental disorders (OR: 3.46, 95 % CI: 2.07-5.82), experience of domestic violence (OR: 10.54, 95 % CI: 1.54-20.68) and medical issues (OR: 2.16, 95 % CI: 1.14-4.03). Limitations: Pre-pandemic data were not available and the sample was recruited mainly by snowball sampling.Conclusion: This study revealed subtle differences in the evolution of symptom trajectories during the first year of the Covid-19 pandemic, and highlighted several characteristics associated with the two clusters.Read less <