Background: Our objective was to evaluate the effectiveness and cost-effectiveness of the positive psychology intervention 'Happiness Route' compared to an active control condition in a vulnerable population with an accumulation of health and psychosocial problems. Methods: We conducted a randomized, single-blind, actively-controlled, parallel group study in seven municipalities in the Netherlands. To be eligible, participants had to experience loneliness, health problems and low socio-economic status. Each group received several home visits by a counsellor (two in the control condition, two to six in the experimental condition). In the Happiness Route, a happiness-based approach was used, whereas the control condition used a traditional problem-based approach. The primary outcome was well-being, measured with the Mental Health Continuum-Short Form (MHC-SF). Results: Fifty-eight participants were randomized to the Happiness Route, 50 to the control condition. Participants were severely lonely, had on average three health problems and less than 5% had paid work. The total MHC-SF score, emotional and social well-being, depression and loneliness improved significantly over the nine-month period in both conditions (p <.05), but there were no significant changes between the conditions across time. Languishing decreased significantly from 33% at baseline to 16% at follow-up among the Happiness Route participants but did not change significantly in the control condition. No significant improvement over time was found in psychological well-being, resilience, purpose in life, health-related quality of life and social participation. Cost-effectiveness analysis showed that expected saved costs per QALY lost was €219,948 for the Happiness Route, relative to the control condition. The probability was 83% that the Happiness Route was cost saving and 54% that the Happiness Route was cost-effective at a willingness to accept a threshold of €100,000. Conclusions: Mental health status of both groups improved considerably. However, we could not demonstrate that the Happiness Route yielded better health outcomes compared to the control condition. Nevertheless, the results of the cost-effectiveness analysis suggested that the Happiness Route is an acceptable intervention from a health-economic point of view. Our results should be viewed in light of the fact that we could not include the planned number of participants. Trial registration: Netherlands Trial Register: NTR3377. Registered 2 Apr 2012.
