A heterologous boosting with SX HA DNA vaccine protected mice against lethal challenge by the QH H5N1 influenza virus.

<p>(<b>a</b>) Schematic diagram of the immunization and challenge studies. (<b>b</b>) At 2 weeks after the second immunization, blood samples were collected and analyzed for antibody responses against the QH virus by ELISA, HAI, and virus-neutralization assays. Data are presented as the mean ± standard deviation. Dashed lines indicate detection limit for HAI titer (1∶20) and neutralizing titer (1∶10). 1, Group QH-QH; 2, Group QH-SX; 3, Group SX-QH; 4, Group QH; 5, Group SX; 6, Group SX-SX; 7, Group C. (<b>c</b>) Mouse survival rate and body weight change after lethal influenza virus challenge. At 4 weeks after the second immunization, mice were challenged by intranasal instillation with 100 MLD50 of the QH H5N1 influenza virus and then monitored daily for weight loss and disease progression. Mice that were found to display severe signs of illness or loss more than 25% body weight were sacrificed in accordance with IACUC guidelines.