Potential hazards associated with influenza virus vaccines

There is general agreement that human reassortant vaccine strains should be used only for the preparation of the viral glycoproteins (split vaccine), and not in toto after UV-inactivation. Live vaccines with lowered pathogenicity obtained by reassortment between human and avian strains may carry a risk of causing epizootics in other species, even though they are useful for the host for which they were designed. It has been argued that this dangerous situation is avoided when both parent strains are of human origin. For this reason, cold-adapted human master strains are used for reassortment with the most recent isolates from cases of human influenza. There is, however, convincing evidence that a reassortant between two human strains has caused severe epizootics among camels, which were not regarded as natural hosts for influenza A viruses. This sudden appearance of a reassortant camel virus had a precedent in experiments in which, starting from parent strains that are non-pathogenic for mice, highly pathogenic reassortants for this species were created. Safety requirements for cold-adapted reassortants must therefore take account of the fact that these new strains may have a high pathenogenicity for other species.