We study the effect of an emerging virus mutation on the evolution of an epidemic, inspired by the appearanceof the delta variant of SARS-CoV-2. We show that if the new variant is markedly more infective than theexisting ones the epidemic can resurge immediately. The vaccination of the population plays a crucial role inthe evolution of the epidemic. When the older (and more vulnerable) layers of the population are protected,the new infections concern mainly younger people, resulting into fewer hospitalisations and a reduced stress onthe health system. We study also the effects of vacations, partially effective vaccines and vaccination strategiesbased on epidemic-awareness. An important finding concerns vaccination deniers: their atttude may lead to aprolonged wave of epidemic and an increased number of hospital admissions.