TABLE 1

Rationale of selected substitutions

SubstitutionSubtype no. conversionGenetic difference betweena:Position involved in antigenic change of other influenza virusesb
A(H1N1)pdm09 viruses and swine A(H1N1) virusesA(H1N1)pdm09 viruses and seasonal A(H1N1) virusesSwine A(H1N1) viruses and seasonal A(H1N1) virusesA(H3N2) virusA(H5N1) virusInfluenza B virusSubstitutions associated with changes in receptor binding specificity
84SN
127DE126cf
127DN126cf
127DT126cf
142KN145d23
142KS145d23
152VT151,c 155d2324
153KE156d23
153KH156d23
153KQ156d23
155GE158d, 165e2323
156ND159d23
156NG159d23
156NS159d23
156NY159d23
186AK185,c 189d2324
186AQ185,c 189d2324
190SD189,c 193d2324
190SN189,c 193d2324
222DE9
222DG9
223QL222c10
223QR222c8
224EA9
225GS224c10
152VT156NS
187DE222DG9
155GE224EASP
222DG224EA9
223QL225GS10
  • a Genetic differences were identified from an amino acid alignment of A(H1N1) viruses that circulated between 1918 and 2009, including 11 pre-1957 human and 5 swine A(H1N1) viruses (data not shown). Subtype number conversion is indicated where appropriate.

  • b Numbers refer to previous studies listed in the References. SP, substitution predicted to change receptor binding specificity based on structural modeling (data not shown).

  • c H5.

  • d H3.

  • e Influenza B.

  • f Unpublished results.