evidence: Short-term side effects of COVID-19 vaccines among healthcare workers: a multicenter study in Iran [...] Reactogenicity refers to a subset of reactions that occur soon after vaccination, and are a physical manifestation of the inflammatory response to vaccination. In clinical trials, information on expected signs and symptoms after vaccination is actively sought (or ‘solicited’). These symptoms may include pain, redness, swelling or induration for injected vaccines, and systemic symptoms, such as fever, myalgia, headache, or rash. The broader term ‘safety’ profile refers to all adverse events (AEs) that could potentially be caused/triggered or worsened at any time after vaccination, and includes AEs, such as anaphylactic reactions, diseases diagnosed after vaccination and autoimmune events. [...] Pre-clinical evaluation investigating injection-site reactions and the toxicological profile of vaccines is a prerequisite to the initiation of clinical trials in which safety and efficacy are established. However, while preclinical studies are critical to identify signals that would prevent their use in humans, evaluation in the early phase of clinical development is needed to confirm the safety profile of the vaccine in humans. Systematic collection of safety data in clinical trials follows guidelines set out by competent regulatory agencies, such as the European Medicines Agency, the US Food and Drug Administration and World Health Organization (WHO). Authorities also advise on the size of the safety database needed to provide reasonable assurance that the reactogenicity and safety profile of the vaccine is acceptable.