Fig 1.

Hemagglutinin stalk antibodies are produced following replicative infection. Animals were infected with 10⁴ PFU of Cal09, NC99, or SI06 virus (A), vaccinated with PR8 DNA, NC99 DNA, FM1 inactivated virus, or PR8 inactivated virus (B), or vaccinated with 10³ or 10⁴ PFU live attenuated cold-adapted or NS1-truncated viruses expressing PR8 HA (C). Sera were harvested from mice 4 weeks after infection or vaccination, and hemagglutinin stalk-specific antibodies were assayed by ELISA using cH6/1 protein. Serum raised from mice vaccinated with headless hemagglutinin and monoclonal antibody 6F12 was used as a positive control.