# Comparing tailored implementation strategies to improve intervention fidelity in a school-based obesity prevention program: the IMPROVE hybrid type III trial

## TL;DR

This study compared two implementation strategies for a school-based obesity prevention program in Sweden and found that adding more strategies didn't significantly improve results.

## Contribution

The study evaluates the effectiveness of tailored implementation strategies in a real-world obesity prevention program.

## Key findings

- Enhanced implementation strategies did not significantly improve adherence compared to basic strategies.
- Parents from Nordic countries were more likely to complete health talks than those from outside the region.
- Contextual factors like time and interest affected the success of implementation strategies.

## Abstract

In Sweden, childhood overweight and obesity rates have risen significantly over the last decades, necessitating scalable interventions. The evidence-based Healthy School Start (HSS) program integrates school and family components to promote healthy habits and prevent overweight and obesity among children. The IMPROVE trial aimed to compare the effect of two tailored implementation strategy bundles (Basic and Enhanced) on fidelity to the HSS program.

A hybrid type III cluster-randomized trial with two parallel arms was conducted in 45 schools (cluster) in three municipalities in Stockholm Sweden from August 2021 to June 2024. The program was implemented in two consecutive cohorts over two academic school years. Fidelity was measured with an adherence score (0–4) and parent’s responsiveness (1–5) to the four intervention components (health brochure, motivational interviewing health talk, classroom module and type 2 diabetes risk test). Data were analyzed using mixed-effects linear and logistic regression models.

A total of 946 parents and 655 children participated. Overall fidelity, assessed as an adherence score, was around 75%, with most components implemented as expected. The adherence score in the Basic bundle showed no significant difference compared to the Enhanced implementation strategy bundle (β = 0.01, p = 0.95, 95% CI: –0.24, 0.25). Two of four Enhanced implementation strategies, educational outreach visits and networking between school and primary health care, did not happen mainly due to lack of interest and time among personnel. Parents born within the Nordic countries had twice the odds (p < 0.001, 95% CI: 1.14–3.43) of completing the motivational interviewing health talk compared to those born outside the Nordics.

Enhancing the Basic implementation bundle with additional strategies did not consistently improve adherence or responsiveness. However, improvements observed over time underscore the importance of targeted support during the initial implementation year. Additional motivational actions might be needed in schools with a high proportion of children whose parents are born outside the Nordic region. These findings highlight the complex interplay between context and implementation success, emphasizing the need to adapt strategies over time to optimize their effectiveness rather than merely adding more. Moreover, the essentially null findings also point to broader methodological challenges in implementation science, particularly how to prioritize among determinants, strategy selection and tailoring.

ClinicalTrials.gov, Unique Protocol ID: NCT04984421. Registered July 30, 2021, https://register.clinicaltrials.gov/.

The online version contains supplementary material available at 10.1186/s13012-025-01481-0.