Most interventions targeted at-risk youth, not those reporting loneliness. “Very few interventions actually targeted those who reported loneliness.”

https://www.acamh.org/research-digest/interventions-for-reducing-loneliness-seem-effective-in-young-people
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Interventions for reducing loneliness seem effective in young people

Meta-analyses of interventions aimed at reducing loneliness among young people are distinctly lacking in the field. Now, Alice Eccles and Pamela Qualter have addressed this gap by compiling a review for Child and Adolescent Mental Health on interventions to reduce loneliness in young people.

ACAMH