suggesting that S-OIV shares similar antigenicity with those earlier viruses. According
to Itoh et al,9 people infected with the 1918 pandemic virus or closely related human H1N1 viruses elicited neutralizing antibodies to S-OIVs. In another study,39 postinfection antisera from ferrets against current seasonal human H1N1 viruses did not react with S-OIVs.