Maternal HIV viral-load monitoring intervals and reproductive health outcomes in Zambian prevention-of-mother-to-child-transmission programmes: a target-trial emulation of testing-frequency effects on vertical transmission

The frequency of maternal HIV viral-load monitoring in Zambian prevention-of-mother-to-child-transmission (PMTCT) programmes is a modifiable health-system parameter, yet its causal effect on vertical transmission cannot be estimated from routine observational data because monitoring intervals are assigned in response to prior results, clinical risk and clinic capacity. Because no empirical cohort is analysed here, the article characterises the operating characteristics of the proposed design under a declared variance structure: between-cluster standard deviation 6.0, residual standard deviation 4.5 and intraclass correlation 0.64. 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. Under this variance structure, 56 clusters are required to detect a 5-percentage-point effect at 80% power. The framework clarifies what a Zambian PMTCT monitoring-frequency trial would need to estimate, what it would assume, and where its conclusions would remain conditional on unverifiable exchangeability assumptions.