Cloned, i.e. defined, defective interfering (DI) influenza A virus is produced in embryonated hens eggs using a method which generates large quantities of DI virus material. Cloned DI virus is then used in tests on mice and ferrets given a lethal challenge of wild-type influenza A virus. When cloned DI influenza A virus is co-administered with a lethal dose of virulent influenza A virus, mice are protected compared to a control of inactivated cloned DI influenza A virus. Mice which survived the administration of cloned DI influenza A virus and infective challenge virus are three weeks later still protected against lethal challenge with infective virus. Control mice which received only cloned DI influenza A virus and no lethal challenge are not protected three weeks later on lethal challenge with infective virus.