When a side effect of a vaccine is suspected, patients and healthcare professionals have the responsibility to report it to competent agencies such as the Croatian Agency for Medicines and Medical Products in order to contribute to a wider understanding of the potential risks and benefits of vaccines. Research objectives: The main goal was to determine whether healthcare workers report suspected vaccine side effects to the Croatian Agency for Medicines and Medical Products, and to examine whether patients report suspected vaccine side effects to the Croatian Agency for Medicines and Medical Products and, if not, for what reasons, and to analyze the proportion of reported and unreported cases suspected vaccine side effect. Respondents and methods: Data were collected via an online anonymous questionnaire on the Facebook social network in the period from 01.02.2023. until 15.04.2023. years. A study was conducted on 275 adults of both sexes who were vaccinated and/or had vaccinated their child/children. Results: 90.76% of the respondents stated that the healthcare worker did not report the suspected side effect of the vaccine to the competent institutions. 88.21% of respondents did not report suspected vaccine side effects to the Croatian Agency for Medicines and Medical Products, and the most common reasons for not reporting by respondents were: "I didn't know I could do it myself" in 39.18% of cases and "a healthcare worker told me that it is an expected side effect that should not be reported" in 19.30% of cases. Of the total number of suspected vaccine side effects, 11.79% were reported to the Croatian Agency for Medicines and Medical Products, while 88.21% of suspected vaccine side effects remained unreported. Conclusion: Improving patient education, communication, trust and empowerment may be key to encouraging greater reporting of adverse events and consequently improving vaccine safety.