This project aims to evaluate the relative efficacy of a peer-led intervention based on the Social Cognitive Theory and Health Belief Model versus the control (only pamphlet delivery) on increasing the prevalence of healthy oral behaviors and oral health status among adolescents in Hong Kong. A two-arm non-blinded randomized controlled trial will be conducted among random samples of adolescents enrolling in secondary schools in Hong Kong. In addition to a baseline survey, two follow-up evaluative surveys will be conducted at months 6 and 12. Survey questionnaires will be conducted to participants at three time points (baseline, 6-, 12-month follow-up). Dental check-up examinations will be evaluated at two time points (baseline, 6-month follow-up). Repeated measures will be used to record participants' background, oral behaviors, theory based cognition, and potential confounders. Validated scales will be used. DMFT, plaque index and CPI index will be used to assess the oral health status of participants.

Baseline survey and dental examination The baseline survey will collect information on participants' background characteristics (e.g., socio-demographics), Social Cognitive Theory (SCT)-related variables (perceived susceptibility, perceived severity, perceived benefits, perceived barriers, self-efficacy), Health Belief Model (HBM)-related variables (reciprocal determinism, behavioral capability, outcome expectations), and oral health behaviors. Validated scales have been identified and will be used in the survey.

To comprehensively assess each participant's oral health status at baseline, their oral hygiene status, caries experience and periodontal condition will be recorded following the recommendations of the World Health Organization (WHO) for oral health surveys. Oral hygiene status will be recorded through plaque index (PI) (Quigley Hein, 1972) on a scale of 0 to 5. Dental caries experience will be recorded by counting the numbers of permanent teeth that are decayed (DT), missing due to caries (MT), and filled (FT) to generate a DMFT score. Periodontal condition will be recorded using the Community Periodontal Index (CPI). Two trained and calibrated examiners (kappa = 0.81 - 0.87 for intra- and inter-examiner reliabilities for the assessment of DMFT and CPI) will perform the clinical examinations using a plane disposable plane intra-oral mirror with a built-in LED light source and a WHO CPI periodontal probe. Ten percent of the participants will be randomly selected and re-examined in each school to continuously monitor intra- and inter-examiner reliabilities of assessing PI DMFT and CPI.

Intervention description A total of five peer-led activities will be conducted in the intervention schools by the well-trained peer leaders within a 6-month intervention period.

1. In the 1st month of intervention, peer leaders will deliver a poster and a leaflet providing information on healthy oral behaviors based on SCT and HBM. Such materials will be provided at the entrance of the school gate, the canteen, and each class. Peer leaders will encourage students to ask questions when they receive such materials. Participants are free to take an on-site quiz to check assess their knowledge level.
2. In the 2nd month of intervention, peer leaders will give a specific health talk aiming to raise the awareness of healthy oral behaviors. Schools teachers will encourage all students to participate in this talk. We will video-record this health talk and make it available online to reach more students. This talk will be guided by SCT and HBM, and it will include short testimonials of the good experiences of the peer leaders when performing healthy oral behaviors.
3. In the 3rd month of intervention, peer leaders will organize a workshop aiming to help the participants practicing skills in performing healthy oral behaviors. Regarding different oral behaviors (tooth brushing, dental flossing, and dental visits), targeted skills will be practiced.
4. In the 4th month of intervention, peer leaders are encouraged to self-design and conduct one more oral health-related activity that they are interested. Necessary support will be provided by the team. We will record details of this peer leader-initiated activity.
5. In the 5th or 6th month of intervention, peer leaders will deliver a special talk regarding illness representations for students with existing oral diseases only. The talk will focus on improving illness representation in both cognitive and emotional aspects, and coping strategies will be practiced.

The first follow-up at months 6 The first follow-up at months 6 will include two parts. The first part is to collect participants' self-reported data through questionnaire. An experienced fieldworker will facilitate the survey process, and she will be blind to our group allocation. The month 6 follow-up data suggest an immediate impact of the intervention, as all intervention activities will be conducted within this 6-month intervention period. The proposed time interval (6 months) is reasonable and feasible, as 6 months is the most frequently adopted time frame to assess behavior changes and the dental visit for check-up (one of our primary outcomes) is recommended to adolescents every 6 months. We will compare changes before and after this intervention within interventions group, and we will also investigate the differences between intervention group and control group.

The second follow-up at months 12 The second follow-up at months 12 will include two parts. The first part is to collect participants' self-reported data through survey, the same procedure as the first follow-up at month 6. The month 12 follow up data suggest a prolonged impact of the intervention, as no intervention activities will be conducted between months 6 and months 12.

The second part is to do dental re-examination, using the same methods and criteria as the baseline examination. We will compare the disease status changes before and after intervention within interventions group. The consistency between self-reported healthy oral behaviors and dental examination outcomes will also be calculated. No dental examination will be conducted at month 6 follow up, as oral disease status is not likely to change within a short time period like 6 months.

Data analysis The intra- and inter-examiner reliabilities of assessing PI, DMFT and CPI at the tooth level will be tested through the use of the kappa statistic. Data of DMFT and CPI at the tooth level will be used to generate the oral disease status at the subject level.

Intention-to-treat analysis will be conducted. Multiple imputation methods will be used to deal with missing data if it occurs. Between-group (intervention versus control) baseline differences in the frequency distributions of potential confounders will be compared by using chi-square test, t-test, Mann-Whitney test or other statistics. The absolute and relative risks and number needed to treat (and their 95% confidence intervals) for the binary outcomes comparing the two groups at months 6 and 12 will be derived. Adjusted comparisons between the intervention and control groups will be made for all binary outcomes, adjusting for any potential confounders showing p<.10 in the between-group baseline comparisons (if any), using modified Poisson regression with robust sandwich variance estimation. SPSS will be used for data analysis; p<.05 (2-sided) will be taken as statistically significant.

Inclusion Criteria:

• Grade 2 students of Secondary schools at the baseline recruitment
• Agree to voluntarily participate
• Do not intend to leave Hong Kong within the next 12 months

Exclusion Criteria:

• undergoing orthondontic treatment

.An experienced fieldworker will facilitate the survey process, and she will be blind to our group allocation. Two experienced dentists will conduct the dental examinations who don't know the group allocation.