Abstract Background A large measles outbreak started in Romania in 2016. Current study aimed to (i) clarify who was affected by the outbreak, (ii) identify their barriers and drivers to vaccination and (iii) explore variation by population group. Methods This was a two-component study. Outbreak surveillance data for 6743 measles cases were reviewed to identify key characteristics. A survey was administered via telephone to 704 caregivers of measles cases (520 respondents) to explore capability, opportunity and motivation barriers to vaccination. Data were summarized descriptively for respondent characteristics and statements. Differences by population group (education, household income, ethnicity, setting and mobility) were explored using χ2 tests, Fisher’s exact tests or regression models. Results Most cases were unvaccinated and lived in low coverage areas. Ethnic minorities were disproportionally affected. Most caregivers felt welcome at health facilities. Some were less satisfied with the waiting time and had found the vaccine out of stock. Not everybody knew that vaccines were free of charge. Less than half knew the child’s next vaccination date, some had not been informed and did not know where to seek this information. Some said their peers did not vaccinate. Beliefs were generally supportive of vaccination; but many were concerned about vaccine safety and found they had not received good information about this. Conclusions varied greatly between minorities and less educated groups, compared with people with higher education levels. Conclusions Identifying characteristics of the population affected and underlying factors can inform a strategy to avoid future outbreaks and further research to obtain deeper insights.