GENES FROM LIVE VACCINES. “Importantly, the pathogenic consequences of recombination need to be carefully considered whenever multivalent live-attenuated vaccines are used. For example, recombination of poliovirus vaccine strains leads to the frequent recovery of recombinant pathogenic poliovirus; similarly, this is also observed for infectious bronchitis virus.”
http://www.sciencedirect.com/science/article/pii/S0042682215001580