Table 1.

Minimal evolutionary steps to acquire all 34 pandemic markers.

Initial strain	Region	Shift	Drift
H1N1 swine	Henan/Tianjin	H5, N1	199 PB2 117 NS1
H1N1 human	New Zealand	H9, N2	211 PB1
	Australia	H7, N2	117 NS1
	U.S.A., Asia	H5, N1	(one or both)

First column shows the initial strain, the second column shows region where strain is found, the third column shows double reassortments taken (Shift) and column four shows the mutations (Drift) taken. The human case (row 2) involves three subtypes (H9N2, H7N2, and H5N1) and one or two mutations.