Generation and characterization of a cold-adapted influenza A H9N2 reassortant as a live pandemic influenza virus vaccine candidate.

H9N2 subtype influenza A viruses have been identified in avian species worldwide and were isolated from humans in 1999, raising concerns about their pandemic potential and prompting the development of candidate vaccines to protect humans against this subtype of influenza A virus. Reassortant H1N1 and H3N2 human influenza A viruses with the internal genes of the influenza A/Ann Arbor/6/60 (H2N2) (AA) cold-adapted (ca) virus have proven to be attenuated and safe as live virus vaccines in humans. Using classical genetic reassortment, we generated a reassortant virus (G9/AA ca) that contains the hemagglutinin and neuraminidase genes from influenza A/chicken/Hong Kong/G9/97 (H9N2) (G9) and six internal gene segments from the AA ca virus. When administered intranasally, the reassortant virus was immunogenic and protected mice from subsequent challenge with wild-type H9N2 viruses, although it was restricted in replication in the respiratory tract of mice. The G9/AA ca virus bears properties that are desirable in a vaccine for humans and is available for clinical evaluation and use, should the need arise.