H7N9 antibody in Vietnam.

A study by MF Boni and colleagues describe the presence of either specific, or cross-reactive antibodies (which can arise after infection by a different influenza virus, but react with the H7N9 test) to the avian influenza A(H7N9) virus. These antibodies were detected in the sera of a cross-section of people from south Vietnam and were found at higher titres (levels) than antibodies to H5 but below those to H9 viruses. Human seasonal influenzavirus antibody levels were much higher than avian levels.
The protein array method used may not be comparable to haemagglutination inhibition (HI) or neutralization assay.
Results revealed a general increase in the amount of anti-haemagglutinin (HA) IgG antibody with age. This is fairly normal as our exposures to virus accumulate an immune "memory" over time.

There was no significant difference whether the samples came from rural or urban locales or from areas known to often keep chooks in the backyard, versus areas that do not.

The H7N7 and H7N9 avian influenza viruses differ by only ten amino acids in HA so it is likely IgG to H7N7 will cross-react with the H7N9.