The changes G186V and V226I increased egg viral replica-tion of A/Fujian/411/02(H3N2) viruses so did the changes G186V and A196T for A/California/20/ 99(H3N2) viruses [56, 57] . On the contrary, others have stated that the V226I change in combination with T155 and H156 do not result in viral recovery in eggs [37, 54] . This might explain the delay in the 2006-2007 vaccine production for the northern hemisphere due to egg propagation difficulties with the A/Wisconsin/67/05(H3N2) strain. The influence on replication efficiency by the other substitutions observed at receptor binding sites should be investigated further.