Homotypic and heterotypic immunity of influenza A viruses induced by recombinants of the cold-adapted master strain A/Ann Arbor/6/60-ca.

The cold-adapted (ca) influenza A virus A/Ann Arbor/6/60-ca when administered intranasally to mice in two doses 3 weeks apart induces solid immunity to challenge 3 weeks later with heterotypic influenza A wild-type viruses (17). In the present study heterotypic immunity against viruses from different sub-types was shown to be relatively short-lived, having declined significantly 9 weeks after vaccination and being completely absent by 21 weeks. On the other hand, immunity against challenge viruses with surface antigens similar to the ca vaccinating virus or to other H 3 N 2 viruses remained high, even in the absence of detectable serum haemagglutination-inhibiting antibody. Both short- and long-term immunity induced by ca viruses was unaffected by earlier priming experiences with other wild-type or ca viruses. These results suggest that at least two mechanisms are involved in respiratory immunity to influenza viruses.