1. High titres of neuraminidase-inhibiting antibody were detected in convalescent human sera following natural influenza A2 infections.
2. Such antibody was encountered infrequently in acute serum samples. Antibody persisted only 5–6 months after infection in the four individuals from whom serial serum specimens were available.
3. Following immunization with killed influenza virus vaccines (with adjuvant) neuraminidase inhibiting antibody was detected in human sera. The titres were in general lower than those detected in convalescent human sera.
4. The specificity of the neuraminidase-inhibiting antibody in human and animal antisera was studied. Tests with convalescent human sera using purified neuraminidase preparations and with a recombinant virus containing A2 neuraminidase and haemagglutinin distinct from that of human influenza A viruses enabled the conclusion that the antibody detected was specific for influenza A 2 neuraminidase.
We wish to thank Dr D. A. J. Tyrrell of the Clinical Research Centre, Mill Hill, London, for human serum specimens, Dr D. Breeze of Evans Medical Ltd., Liverpool, for sera from an influenza vaccine trial and Dr M. S. Pereira of the Public Health Laboratory, Colindale, London, for serial serum samples from persons who had influenza in 1957. Dr M. S. Pereira carried out the haemagglutination-inhibition tests and complement fixation tests on these sera.
We are grateful to Dr H. G. Pereira and Dr D. A. J. Tyrrell for valuable discussions during the course of this study, and to Professor C. H. Stuart-Harris for his comments on the manuscript.