T-cell epitopes are more conserved than antibody epitopes. Fifteen per cent of the T-cell epitopes are conserved while only 2.7% of the antibody epitopes [48] . The reason for this higher degree of conservation is that 80% of the antibody epitopes are located in the variable glycoproteins HA and NA, while only 40% of the T-cell epitopes are found in these proteins [48] . Recent research has shown some degree of escape from CTL-mediated immunity in addition to escape from neutralizing antibodies [28] . In our dataset we found several substitutions in regions involved in protective T-cell response [48] in NA, PA, M1, NS1 and most in the NP protein. This is not unexpected because most T-cell epitopes are defined for the NP protein and this protein is the main target for the cytotoxic host immune response [49, 50] . The extensive variations in the T-cell epitopes during 1999 to 2006 suggest that these regions and the antibody epitopes are working together for efficient escape from the host defence responses.