Background: The COVID-19 pandemic negatively impacted global mental health, with adolescents experiencing disproportionate effects. Limited research has explored the impact of different pandemic restrictions on adolescent mental health, and only a few studies have examined the longer-term impacts of the pandemic on adolescent mental health. These investigations are crucial for informing public health policies, particularly the integration of mental health care in future public health emergencies.

Methods: This study aimed to investigate the impact of lockdown duration and the impact of adolescents' subjective experiences of the pandemic on their wellbeing, internalising symptoms, and externalising symptoms. Australian adolescents (N = 1,001, mean age = 14.2 years) completed a baseline survey in 2021, shortly after pandemic lockdowns were lifted (Time 1), and a follow-up survey approximately 12 months later (Time 2). Predictors of interest were the total duration of COVID-19 lockdowns across 2020-2021, and adolescents' subjective experiences of the pandemic on their social connections, learning, technology use and family relationships. A range of covariates were included in analyses to examine subgroup differences.

Results: Linear mixed-effects models indicated that total duration of the lockdown was not associated with any of the outcomes at Time 1 or Time 2 (all ps > 0.017). Negative subjective experience of the pandemic on learning was associated with greater externalising symptoms at both Time 1 (t = 5.17, df = 980, p <.001) and Time 2 (t = 2.72, df = 708, p =.007). Negative subjective experience of the pandemic on social connection was associated with greater internalising symptoms at Time 2 only (t = 3.20, df = 709, p =.001). Negative subjective experience of the pandemic on family relationships or technology use was not associated with any of the outcomes at Time 1 or Time 2 (all ps > 0.017).

Conclusions: Adolescents' negative subjective experience of the pandemic on learning and social connections was associated with greater internalising and externalising symptoms after the lockdown had been lifted. Duration of lockdowns was not associated with any of the primary outcomes. During future public health emergencies, mental health interventions should be tailored to assist adolescents to adapt to new learning environments, and to build and maintain social connections.