The circulating type A (H3N2) viruses in the first year of the study (2004–2005) were considered
to show moderate antigenic drift from the vaccine strain (differences in dilutions by a factor
of eight in cross hemagglutination-inhibition assays with the use of ferret antiserum).9,19 In 2007–2008, the antigenic differences between the A/Wisconsin vaccine strain and the A/Brisbane circulating strain were found to be twofold with
the use of the same assays, indicating a low level of drift.12 Despite these differences in the degree
of drift between vaccine and circulating type A (H3N2) strains during the study years 2004–2005 and 2007–2008, estimates of efficacy for the inactivated vaccine were similar