Compositions and methods for diagnosing and/or treating influenza infection

A method of stratifying a patient population comprising:

• providing a patient population;
• contacting blood from each of the patients in the population with a binding agent;
• determining whether the binding agent binds to an H1 HA polypeptide in each patient's blood;
• and
• determining that, if the binding agent binds to an H1 HA polypeptide in a patient's blood, that:
• the patient will be protected against an influenza strain demonstrating enhanced human binding or infectivity influenza upon administration of a vaccination against A/South Carolina/1/1918 or A/Swine/Iowa/15/1930;
• or
• the patient will be less susceptible to infection by an influenza strain demonstrating enhanced human binding or infectivity;
• or
• the patient will demonstrate a lesser need of vaccination or treatment than patients not previously exposed.

An engineered H1 HA polypeptide:

• whose amino acid sequence further includes a residue at a position corresponding to residue 186 of wild type H3 HA, which residue in H1 HA is not Ser;
• whose amino acid sequence further includes a residue at a position corresponding to residue 189 of wild type H3 HA, which residue in H1 HA is not Pro or Ala;
• and
• whose amino acid sequence further includes a residue at a position corresponding to residue 227 of wild type H3 HA, which residue in H1 HA is not Thr or Glu.