The present invention relates to pneumoviral virions comprising a viral genome that has a mutation in a gene coding for a protein that is essential for infectivity of the pneumovirus, whereby the mutation causes a virus produced from only the viral genome to lack infectivity, and whereby the virion comprises the protein in a form and in an amount that is required for infectivity of the virion. The invention for relates to methods for producing the pneumoviral virions and for using the virions in the treatment or prevention of pneumoviral infection and disease. A preferred pneumoviral virion is a virion of Respiratory Syncytial Virus in which preferably the gene for the G attachment protein is inactivated and complemented in trans.
