Antigenic reactivity of pandemic influenza (H1N1) 2009 viruses from UK (n = 227).

Viruses are grouped according with amino acid signature changes described in Figures 2 and 3. Substitutions located within the signal peptide were not considered.aAmino acid changes are described using as reference the sequence of A/California/07/09.bRefers to 4-fold dilutions of antiserum against A/California/07/09. Viruses with >4-fold reduced antigenic reactivity are considered as “low reactors”. Compared with the vaccine strain antiserum, the titres with A/England/195/2009 antiserum were within one-fold of difference in most cases and all the pandemic viruses showed no reactivity against A/Brisbane/59/07 antiserum; therefore these results are not included in the analysis.cGroups where >50% of the viruses shows ≥4-fold reduced reactivity are highlighted in bold letter.