FIG 4.

HAI serum antibody titers induced by two H1N1 infections and one VLP vaccination. Ferrets were infected with Sing/86 and 84 days later with NC/99 or Bris/07 virus. At day 168, these preimmune ferrets (n = 4) were vaccinated with VLP vaccines expressing P1 HA (A), X6 HA (B), or CA/09 HA (C), or they were mock vaccinated (D). At day 252, serum samples were collected, and the HAI titers were determined for each group ferrets against a panel of 15 H1N1 influenza viruses isolated between 1934 and 2013. Values are geometric mean titers plus the SEM (error bars). The dotted lines indicate the 1:40 HAI titer.