Here’s a highly relevant, recently published paper:

Lasgaard M, Qualter P, Løvschall C, et al. Are loneliness interventions effective for reducing loneliness? A meta-analytic review of 280 studies. Am Psychol. Published online October 23, 2025. doi:10.1037/amp0001578

If you know German, you might be interested in the two books on loneliness published by Noëmi Seewer and Tobias Krieger.