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Human vaccines against influenza have been available for almost 60 years and, until recently, were prepared almost entirely from viruses grown in the allantoic cavity of 9- to 11-day-old embryonated chicken eggs. Manufacture involving eggs is not sufficiently flexible to allow vaccine supplies to be rapidly expanded, especially in the face of an impending pandemic. Other problems may arise from the infections of progenitor flocks that adversely affect egg supplies, and from the manufacturing process itself, where breakdowns in sterility can occur from the occasional contamination of large batches of viral allantoic fluid. In addition, egg-grown viruses exhibit differences in antigenicity from viruses isolated in mammalian cell lines from clinical specimens. These concerns and the probable need for greatly expanded manufacturing capability in the future have been brought into focus in recent years by the limited spread of H5N1 avian influenza infections to humans in several Asian countries. Alternative approaches involving the use of accredited anchorage-dependent and -independent preparations of the African Green monkey kidney (Vero), Madin-Darby canine kidney (MDCK) and other cell lines have been pursued by several manufacturers in recent years. Yields comparable with those obtained in embryonated eggs have been achieved. These improvements have occurred in parallel with newer technologies that allow the growth of cells in newer synthetic media that do not contain animal serum, in order to allay the concerns of regulators about the potential for spread of transmissible spongiform encephalopathies. [ABSTRACT FROM AUTHOR]

The growth of the Russian live attenuated influenza vaccine donor strains A/Leningrad/134/17/57, A/Leningrad/134/47/57 and B/USSR/60/69 was studied in cells of the VERO and Madin-Darby canine kidney (MDCK) lines as six-well cultures and cell factories infected at different multiplicities of infection. Yields for A/Leningrad/134/17/57 and A/Leningrad/134/47/57 were comparable in either cell line over a range of multiplicities but were about 10-fold lower than in the allantoic fluids of infected chicken embryos. For both A/Leningrad/134/47/57 and B/USSR/60/69, yields from the MDCK line were about 10-fold higher than for the VERO line. For B/USSR/60/69, yields in eggs were approximately 100-fold higher than those obtained in the MDCK line. A feature of the growth of B/USSR/60/69 was its reduced capacity to produce infectious progeny in either cell line at multiplicities of infection of 2.0 or 1.0 pfu/cell. Inhibition was due probably due to the presence of defective-interfering particles and was not detected with A/Leningrad/134/17/57 or A/Leningrad/134/47/57 in cultures of either line infected at the same multiplicities. Yields for both A/Leningrad/134/47/57 and B/USSR/60/69 in cells of the MDCK line were comparable when grown in six-well cultures or cell factories.

Pandemic influenza A viruses of avian origin are of particular concern and have crossed the species barrier several times in recent years, giving rise to illness and occasionally death in humans. This situation could become dramatically worse if the infectivity of avian viruses for humans were increased by reassortment between the genes of human and avian viruses. Co-infection of humans or an intermediate host with an avian strain and an existing human strain could produce new viruses of unknown pathogenicity to which the entire population would be susceptible. Inactivated vaccines against influenza have been prepared for many years using viruses grown in embryonated chicken eggs. However, the use of eggs presents difficulties when vaccine supplies need to be expanded at short notice. It seems likely that future vaccines will be prepared in high-yielding cell cultures from continuous lines that are preferably anchorage-independent. At present, only certain preparations of the Vero and Madin-Darby canine kidney cell lines, grown and maintained in serum-free medium, are acceptable to all regulatory authorities. However, this situation is likely to change with increasing need for non-pandemic and pandemic vaccines.