Developing a vaccine against several diseases remains a challenge. Several vaccine candidates are in clinical stages of development. The pre-clinical development stage is the first step to identify a vaccine candidate that determines a vaccine’s ultimate safety profile. Phase I trials is the stage where the researchers test the candidate vaccine for the first time in humans to evaluate its safety, safe dosage range, and vaccine-related side effects. Phase II trials is the stage where the candidate vaccine is administered to a larger group of people for the evaluation of its immunogenicity and safety. The goal of phase III is to conduct a large-scale safety and efficacy study on a relevant patient population. It is really very complex to market a new vaccine and once it has been marketed, pharmacovigilance activities take place in order to conduct a strict safety supervision of that vaccine.