ATTENUATION OF HUMAN INFLUENZA A VIRUSES

The attenuation of two human influenza A viruses has been carried out, using the selection of inhibitor-resistant strains and multiple passages at low temperatures. A virus related to A2/Tokyo/3/67 was obtained in an inhibitor-resistant form. When this was compared with the inhibitor-sensitive strain in a volunteer trial it was relatively non-pathogenic. The second virus, A2/Hongkong/l/68, was subjected to much longer treatment, but nevertheless remained slightly sensitive to serum inhibitor. When given to volunteers it was less pathogenic than before but attenuation was incomplete. A2/Hongkong/l/68 was also modified by passage at low temperatures. Many of these passages are apparently necessary for full attenuation. All attenuated viruses were infective and antigenic. © 1969, British Medical Journal Publishing Group. All rights reserved.