Antisera from mice vaccinated with the PR8 4G headless HA show broad cross-reactivity by ELISA. The vaccine groups from which sera are derived are identified at the top of each column, and the ELISA substrate used is indicated to the right of each row. The colors used are consistent throughout the figure, with sera from Gag-only-vaccinated mice indicated in black, HK68 4G-vaccinated mice indicated in green, PR8 4G-vaccinated mice indicated in blue, and full-length-PR8-HA-vaccinated mice indicated in red. Each mouse is represented by a unique symbol that is the same in each panel. A rabbit antiserum raised against whole PR8 virus is shown in gray with open triangles, and a serum sample taken from a naive mouse is shown in gray with open squares. The reactivities of mouse sera to whole PR8 virus (A), purified recombinant A/New Caledonia/20/1999 HA protein (B), purified recombinant A/California/04/2009 HA (C), purified recombinant A/Singapore/1/1957 HA (D), purified recombinant A/Viet Nam/ 1203/2004 HA (E), and purified recombinant A/Hong Kong/ 1/1968 HA (F) are shown.