Healthcare workers (HCWs) are often referred to as the most trusted source of vaccine-related information for their patients. However, the evidence suggests that a number of HCWs are vaccine-hesitant. This study consists of 65 semi-structured interviews with vaccine providers in Croatia, France, Greece, and Romania to investigate concerns HCWs might have about vaccination. The results revealed that vaccine hesitancy is present in all four countries among vaccine providers. The most important concern across all countries was the fear of vaccine side effects. New vaccines were singled out due to perceived lack of testing for vaccine safety and efficacy. Furthermore, while high trust in health authorities was expressed by HCWs, there was also strong mistrust of pharmaceutical companies due to perceived financial interests and lack of communication about side effects. The notion that it is a doctor's responsibility to respond to hesitant patients was reported in all countries. Concerns were also seen to be country- and context-specific. Strategies to improve confidence in vaccines should be adapted to the specific political, social, cultural and economic context of countries. Furthermore, while most interventions focus on education and improving information about vaccine safety, effectiveness, or the need for vaccines, concerns raised in this study identify other determinants of hesitancy that need addressing. The representativeness of the views of the interviewed HCWs must be interpreted with caution. This a qualitative study with a small sample size that included geographical areas where vaccination uptake was lower or where hesitancy was more prevalent and it reflects individual participants' beliefs and attitudes toward the topic. As HCWs have the potential of influencing patient vaccination uptake, it is crucial to improve their confidence in vaccination and engage them in activities targeting vaccine hesitancy among their patients.