Abstract
Background: Preliminary evidence suggests that mental health problems can transmit within adolescent peer social groups. However, prior studies have primarily quantified exposure by counting peers with mental health problems, which cannot precisely reflect the group level density of already affected peers. Moreover, existing evidence predominantly focused on transmission associated with exposure to severe cases, neglecting the more widely prevalent mild-to-moderate cases. Therefore, we explored whether and in what condition exposure to mild-to-moderate cases should be considered along with severe cases in the transmission of mental health problems. Methods: We analyzed data from a mental health monitoring project involving 20 middle schools in Guangdong, China, encompassing all students in 449 classes. The risks of adolescents reporting worse depressive and anxiety symptoms associated with exposure to classmates with severe symptoms or mild-to-severe symptoms were explored through three-level mixed-effect multiple Poisson regression models, adjusted for random effects at classroom and school levels. Findings: Among the 19,058 participants (48.3% girls), 5651 (29.6%) reported depression problems and 6464 (33.9%) reported anxiety problems. Higher risks of adolescents reporting worse depressive and anxiety symptoms were significantly associated with exposure to classmates with any severity of symptoms when considering the percentage of these classmates in the classroom (IRR ranged between 1.01 and 1.02). Conversely, having greater number of classmates with severe symptoms was significantly associated with higher risk of reporting worse symptoms (IRR ranged between 1.03 and 1.09) regardless of proportion of these classmates. Interpretation: Our findings indicated that in group level mental health transmission, the level of exposure should be interpreted with consideration of group density rather than mere number of already affected peers. Moreover, mental health problems can transmit beyond exposure to peers with severe symptoms, thereby facilitating more effective risk screening and prevention of mental health problem transmission in adolescents. This approach is imperative, given the substantial prevalence of mild-to-moderate mental health symptoms among adolescents.