After vaccination, control pigs were bled at day 52, to be tested by SNT to the vaccine strain, with the aim to determine the MDA decay and establish the day of challenge.All pigs were bled just before challenge and sera tested by SNT to a MA-104 adapted subtype 1 strain, with the aim to determine the presence of NA that could be directedto the challenge strain. The challenge strain itself could not be used in this assay due to the fact that, in the testing laboratory, is not adapted to MA-104.