Fig. 3.

Neutralizing activity in vaccinated pig sera. Pigs were vaccinated twice (day 0 and 21 pv1) i.d. with needle-free delivery with 200 μg (n = 6), 800 μg (n = 6) or 1972 μg (n = 5) DNA, or not DNA vaccinated at all (n = 5). The pig sera were tested in a microneutralization assay on days 21, 28 and 35 pv1. Neutralizing antibody titers, MN titers, were evaluated by the capacity of the sera to prevent the infection of MDCK cells by (a) H1N1pdm09, (b) 1999 H1N1 and (c) swine H1N1 isolates. The MN titer was defined as the reciprocal dilution giving 50% infection inhibition and calculated with a linear interpolation method [28]. Serum samples with a titer below the detectable limit of the assay (lowest serum dilution tested was 1:20) were assigned a value of 10 for graphical representation and statistical analyses. Error bars indicate the mean ± SEM, and significant differences from the no-vaccine control group are indicated by: ***: p < 0.001; **: p < 0.01; *: p < 0.05.