The invention relates to the use of a bacterial artificial chromosome (BAC) for the preparation of a vaccine, wherein the BAC comprises an inducible bacterial ori sequence for amplification of the BAC to more than 10 copies per bacterial cell. Plus a viral expression cassette comprising a cDNA of an attenuated RNA virus genome and comprising cis-regulatory elements for transcription of said viral cDNA in mammalian cells and for processing of the transcribed RNA into infectious viral RNA.