Recombinant virus capable of stably expressing target proteins

A recombinant virus comprising an expression cassette in the genome of the recombinant virus, wherein the expression cassette comprises a first polynucleotide encoding at least one polypeptide of interest, and a second polynucleotide which is an essential gene of the recombinant virus or a functional fragment thereof, and wherein the first polynucleotide is functionally linked to the second polynucleotide with the first polynucleotide being located upstream of the second polynucleotide, and wherein said second polynucleotide is the only copy of said essential gene or functional fragment thereof that is active in the recombinant virus.