The role of defective interfering particles in persistent infection of Vero cells by measles virus.

Persistent infections by measles virus were rapidly established in the majority of Vero cells when monolayers were infected with virus stocks that had been passed three to five times from an undiluted inoculum. These virus stocks had low infectivity titres but normal haemagglutinin titres and were able to cause interference. The ability of such virus stocks to establish persistent infections seems to be due to the presence of defective interfering particles rather than of virus mutants. Measles virus released from a persistently infected Vero cell line at the 93rd passage had properties similar to the undiluted passage virus that generated persistent infections.