Claims about vaccine interventions in low-income countries require data from low-income countries: a commentary on Liu et al. (2024)

Liu et al. (2024) recently published a systematic review and meta-analysis of studies on vaccine promotion interventions. One key finding highlighted in the study’s abstract is that strategies that increased access to vaccines were especially effective in lower-income countries and those with limited healthcare access. Here, we raise two issues that call this finding into question. First, Liu et al.’s dataset contains no studies in which access interventions are implemented in low- or even middle-income countries, meaning that any claims about the efficacy of access interventions in these regions are extrapolated from a pattern so far observed only in high-income countries. Second, the interaction effect supporting this conclusion appears to be an artifact of the authors’ selecting a model that systematically excluded the largest non-access intervention effects in low- and middle-income countries.