Measles Supplemental Immunization Activities (SIAs) are resource intensive.
SIAs importantly reach some previously unvaccinated (so-called measles zero-dose) children.
We highlight difficulties in measuring how many measles zero-dose children the SIA reaches.
We also suggest some other methods to improve estimation of SIA impact.
Keywords: Coverage; Equity; Measles; Supplementary immunization activities; Surveys.