The cold-adapted (ca) influenza virus A/Ann Arbor/6/60 (AA)
(H2N2) has been developed as a live attenuated vaccine seed virus
that exhibits cold-adaptation, temperature-sensitivity (ts), and
attenuation (att) phenotypes that are specified by mutations in
the internal genes. Reassortant H1N1 and H3N2 human influenza A viruses with the six internal gene segments of the AA ca virus
bear these phenotypes and extensive evaluation in humans has
proven them to be attenuated and safe (Vesikari et al., 2006).
Accordingly, they have been approved for use as live virus vaccines
in humans. Here, we generated a recombinant H1N1 virus that
bears the HA and NA genes of a 2009 H1N1 virus isolated in China
and the six internal genes from the cold-adapted attenuated virus
A/Ann Arbor/6/60 (AAca). We evaluated its efficacy against homologous
2009 pandemic virus, heterologous H1N1 virus, and H5N1
influenza viruses in mice