This guideline addresses the clinical evaluation of vaccines intended for the prevention of infectious diseases. It includes considerations for trials intended to document the safety, immunogenicity and efficacy of new candidate vaccines and to support changes in the prescribing information of licensed vaccines. It also considers the need for and use of vaccine effectiveness studies. The draft guideline includes specific considerations for clinical trials with vaccines in special populations, such as pregnant women or the elderly. It also adds considerations to priming and boosting strategies, including the option of heterologous prime-boost, which entails administration of one type of vaccine first followed by a different type of vaccine for the same pathogen later.