Fluoride Varnish Application Frequency and Dental Caries Arrest in South African School Hygiene Programmes: A Target-Trial Emulation of Recall Interval Thresholds

School oral-health programmes in South Africa apply fluoride varnish at fixed recall intervals, but the interval that reliably arrests early enamel lesions has not been established against the clustered structure of school-level delivery. the present analysis treats the recall interval as a design parameter and characterises the operating characteristics of a before-after-control-impact (BACI) estimator for the varnish-frequency question under a declared variance structure. Using a between-cluster standard deviation of 6.0, a residual standard deviation of 4.5 and an intraclass correlation of 0.640, power was estimated across 2000 replications per design cell for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The analysis indicates that 52 clusters are required to detect a 5-percentage-point difference at 80% power, and that the minimum detectable effect falls steeply as cluster count rises from 8 to 32. The contribution is a design map that converts a clinical question about varnish frequency into a sampling question about school clusters, and that shows why recall-interval trials in this setting are usually underpowered rather than inconclusive.