Breadth of influenza A antibody cross-reactivity varies by virus isolation interval and subtype.

Influenza virus infections generate antibodies that cross-react with antigenically similar viruses. However, the breadth of cross-reactivity is unclear. Here we analysed 200,000 haemagglutination inhibition (HAI) titrations from ferrets singly infected with A(H3N2) or A(H1N1)pdm09. We identified consistent influenza A virus subtype-specific patterns where breadth of HAI antibody cross-reactivity was associated with isolation time between tested and immunizing viruses. This was independent of virus strains, passage history and genetic mutations. A 6-year interval between virus isolation resulted in minimal HAI titres for A(H3N2), while A(H1N1)pdm09 demonstrated broader cross-reactivity with moderate titre reductions over the same interval. Analysis of HA substitutions revealed more amino acid substitutions between viruses for A(H3N2) and a greater reduction in HAI titres per substitution compared with A(H1N1). Longitudinal analysis of human antisera across age groups suggests that repeat A(H3N2) exposure increased HAI responses within the cross-reactivity range, with greater increases for more recent viruses. These results provide a framework for antigenically evolving pathogens such as influenza viruses.