The purpose of this study was to describe factors strengthening and weakening vaccination competence. The data were collected by focus group and individual interviews with 40 participants, consisting of health-care professionals, students and clients, and were then analysed by content analysis. The results could be classified into four categories: vaccinator professional conduct, education, client conduct and the vaccination environment. Successful client encounters, comprehensive knowledge of vaccinating, adequate education, clients' positive attitude, suitable physical environment and centralization of vaccinations were considered strengthening factors, whereas their opposites weakened vaccination competence. The two most important factors were vaccinator professional conduct and education, and therefore these factors should be considered the most in education, clinical practice and administration. Further research is necessary to determine how well current education, clinical practice and administration support factors strengthening, and help alleviate factors weakening vaccination competence.