The COVID-19 pandemic has burdened disastrous mortality and morbidity rates in society all over the world. While vaccination is one of the most effective immunization methods to control infectious diseases globally, some have avoided receiving the vaccine. We have aimed to investigate the reasons behind the hesitancy of vaccination among healthcare workers. We performed ten semi-structured interviews with volunteered healthcare workers of Rasoul Akram hospital. Then each interview was anonymized, and Braun and Clarke's thematic analysis method was used to analyze the interviews. Our data analysis revealed thirty-eight different codes as reasons for vaccination hesitancy among our interviewees. All these thirty-eight codes were grouped into ten sub-themes, and these sub-themes were further grouped into our four main themes: 1. Fear of side effects, 2. Distrust, 3. Inefficiency, and 4. Non-necessity. Fear of side effects was the most frequent reason that interviewees mentioned. Also, Half of the interviewees mentioned distrust as a reason for COVID-19 vaccine hesitancy. At least once, all interviewees mentioned that they believed vaccination is inefficient. Some interviewees had beliefs and reasons that made them assume vaccination is simply not necessary. Fear of side effects, distrust, inefficiency, and non-necessity were the reasons that our participants refused to get vaccinated. Fear of side effects and distrust were the most common reasons that led to non-vaccination. •Fear of side effects, distrust, inefficiency and non-necessity are the reasons for COVID-19 vaccination hesitancy among HCWs.•Fear of side effects is the main reason for COVID-19 vaccination hesitancy.•unlike other antivaccination movements, the main cause of COVID-19 vaccination hesitancy is not conspiracy theory.