Figure 1: An inactivated 2009 H1N1 vaccine elicits cross-protective antibodies against the 1918 pandemic H1N1 virus in mice.

(a–c) C57B/6 mice (5 weeks old) were immunized twice intramuscularly (at 28 and 14 days before lethal challenge with 300 LD₅₀ of 1918 influenza virus on day 0) with 15 μg of the indicated inactivated virus or with 1918 VLP. The mock-infected group received PBS instead of virus challenge. Mice were 9 weeks old at the time of challenge. (a) Body weights represent the average of n=5 mice per immunization strategy and error bars represent s.d. of mice remaining at each time point. Asterisks below data points represent statistical significant differences in weights, determined by an unpaired Student's t-test (P<0.05), for the no vaccine, Bris/59/07 (H1N1) and Bris/10/07 (H3N2) vaccination groups for days 3–8 (when more than three mice were remaining in each group); and for Cal/04 at the indicated time points, as compared with the mock group. (b) Percentage survival was calculated from the groups above (n=5 mice per group). The log-rank test was used to determine significance (*P<0.05). (c) Viral titres in lungs of vaccinated mice on days 2 (squares) and 4 (circles) after challenge. Titres are shown for two (black arrows) or three mice per group at each time point. Black bars represent the average values, and statistical significance using an unpaired two-tailed Student's t-test (*P<0.05) was determined as compared with the 'no vaccine' control groups.