The A/Fujian/411/02-like(H3N2) viruses did not antigenically match the A/Moscow/10/99(H3N2) strain included in the 2002-2003 vaccine [29, 36] . The HA substitution D144N was however not responsible for the antigenic drift of the Fujian-like viruses. It has been shown that only two amino acid changes, H155T and Q156H, specified the antigenic difference from Moscow-like to Fujian-like [37] , both are located at antigenic site B. The T155 and H156 amino acids have been maintained in all Danish isolates after the introduction of the Fujian-like viruses. With the A/California/7/04(H3N2)-like viruses in 2004-2005, site 145 has changed from K to S in some isolates and to N in others. S145 has been found at this position before 1999 and N145K was the only cluster-difference substitution between isolates from the seasons 1987/ 1989 and 1992/1995 [38] . It was therefore suggested that substitutions at this site alone could have large antigenic effect. Antigenic site A, located in a loop, makes few contacts with the rest of the structure, therefore 144 and 145 may change drastically without influencing on the overall shape of the HA molecule.