Respiratory syncytial virus with a genomic deficiency complemented in trans

A method for producing pneumoviral virions, the virions comprising a viral genome that has a mutation in a gene coding for a protein that is essential for (in vivo) infectivity of the pneumovirus, wherein the mutation causes a virus produced from only the viral genome to lack infectivity, and wherein the virion comprises the protein in a form and in an amount that is required for infectivity of the virion, the method comprising the steps of;

• (a) infecting a culture of a first host cell with a pneumovirus comprising a viral genome that has the mutation, whereby the host cell comprises an expression vector which directs expression in the host cell of the protein in a form and in an amount that is required for infectivity of the virion;
• and,
• (b) recovering of the virions from the infected host cell culture.