The parameters of the PCR were described as following:one cycle of 95◦C for 3 min, followed by thirty cycles of 95◦C for10 s, 57◦C for 30 s and 72◦C for 3 min, and then one cycle of 72◦Cfor 10 min. The linear influenza vector pJY2013 was also amplified using primers pJY2013 F and pJY2013 R and influenza vectorpDP2002 as template.