Combining target trial emulation and qualitative research to understand the effect of health visiting on child hospital admissions in England.

Health visiting is a complex public health intervention in which specialist nurses work with families to support the healthy development of children up to five years of age. Using routinely collected administrative health data, we emulated a target trial to estimate the effect of enhanced health visiting services on potentially avoidable hospital admissions for children born in 10 local areas in England between 2016 and 2019. We found that receiving additional support from the health visiting team in the early weeks of life was associated with an increased odds of a child experiencing a potentially avoidable hospitalisation (OR = 1.28, 95% confidence interval 1.02 to 1.60). Health visiting may encourage families to seek secondary health care, for example by building confidence in public services or heightening parental anxiety about the risks of childhood health conditions. However, qualitative research and sensitivity analyses indicated that our effect estimate may have been subject to residual confounding, selection bias or both. An in-depth understanding of the intervention and the mechanisms through which treatments are assigned is essential for generating valid estimates of causal effects.