Traditionally, vaccination has been used as a preventative strategy, aimed at curbing the impact of a wide range of infectious diseases. Over the past three decades, vaccine development has moved increasingly into the therapeutic area for the treatment of conditions such as cancer and chronic viral infections. To be successful, such vaccines must elicit a strong cellular as well as humoral immune response. Viral vectors, engineered to carry genes for the expression of a suitable immunogenic protein in the recipient, are widely used. Christian Bréchot at TheraVectys examines how a new generation of lentiviral vectors, capable of triggering effective, long-lasting T cell-mediated immunity, is demonstrating preclinical success in both therapeutic and prophylactic vaccine development.