Using this model, we have shown that homologous or heterologous protection against lethal infection of 2009 pandemic H1N1 virus, CA/E3/09, could be achieved by priming with homologous and heterologous isolates including PR8 and X31 viruses and this protection is dependent on CD4 and CD8 T cells [18], and is supported by other related recent studies [19], [20], [21]. This evidence concerns the gene CD8A and infection.