Attempts to increase the yield of influenza virus from stored infected allantoic fluids.

The deposit or precipitate formed after storage at -20 degrees C of allantoic fluids infected with influenza virus contains a considerable amount of the virus. It has been found that by treatment of this material, after thawing, with sodium chloride and subsequent purification of the treated fluid, all the virus originally present can be recovered, thus ensuring against loss of virus and irregular titration results. The authors describe in detail the method developed, which should prove useful in large-scale production of influenza vaccine when the material has to be stored in the deep-freeze for some time.