Subthreshold depression has been previously shown to be as functionally impairing as major depression.  This systematic review found an overall effect size of 0.38 and a number-needed-to-treat (NNT) of 8.4 for psychological (primarily CBT-based) interventions designed to address it. There was insufficient evidence regarding the effectiveness of such therapies for children under 12 and regarding the prevention of onset of major depression.

Cuijpers, P., Pineda, B. S., Ng, M. Y., Weisz, J. R., Muñoz, R. F., Gentili, C., … & Karyotaki, E. (2021). A Meta-analytic Review: Psychological Treatment of Subthreshold Depression in Children and Adolescents. Journal of the American Academy of Child & Adolescent Psychiatry.

https://www.sciencedirect.com/science/article/pii/S0890856721000745