Not knowing WTF a plasmid is, this helps a lot to understand the problem of contamination of a vaccine with these: > A plasmid is a small circular DNA molecule found in bacteria and some other microscopic organisms. Plasmids are physically separate from chromosomal DNA and replicate independently. They typically have a small number of genes — notably, some associated with antibiotic resistance — and can be passed from one cell to another. Scientists use recombinant DNA methods to splice genes that they want to study into a plasmid. When the plasmid copies itself, it also makes copies of the inserted gene. Basically sounds like these things will copy on their own and inhabit cells, basically a third carrier of DNA alongside the normal chromosomes and mitochondria.