The H1N1 influenza pandemic vaccine has been developed from the 
A/California/07/09 (Cal) virus and the well-known high-yield A/Puerto Rico/8/34 
(PR8) virus by classical reassortment and reverse genetics (RG) in eggs. 
Previous studies have suggested that Cal-derived chimeric hemagglutinin (HA) and 
neuraminidase (NA) improve virus yields. However, the cell-based vaccine of the 
H1N1 pandemic virus has been less investigated. RG viruses that contained 
Cal-derived chimeric HA and NA could be rescued in Madin-Darby canine kidney 
cells that expressed α2,6-sialyltransferase (MDCK-SIAT1). The viral growth 
kinetics and chimeric HA and NA properties were analyzed. We attempted to 
generate various RG viruses that contained Cal-derived chimeric HA and NA, but 
half of them could not be rescued in MDCK-SIAT1 cells. When both the 3'- and 
5'-terminal regions of Cal HA viral RNA were replaced with the corresponding 
regions of PR8 HA, the RG viruses were rescued. Our results were largely 
consistent with those of previous studies, in which the N- and C-terminal 
chimeric HA slightly improved virus yield. Importantly, the chimeric HA, 
compared to Cal HA, showed cell fusion ability at a broader pH range, likely due 
to amino acid substitutions in the transmembrane region of HA. The rescued RG 
virus with high virus yield harbored the chimeric HA capable of cell fusion at a 
broader range of pH.