Serum Samples From Middle-aged Adults Vaccinated Annually with Seasonal Influenza Vaccines Cross-neutralize Some Potential Pandemic Influenza Viruses

We examined serum samples from adults ages 48-64 who received multiple seasonal influenza vaccines from 2004 to 2009 for cross-neutralizing antibodies to potential pandemic strains. Using pseudoviruses bearing various hemagglutinins (HA-pseudoviruses), we found serum neutralization titers (>= 160) in 100% against A/Japan/305/1957 (H2N2), 53% against A/Hong Kong/1073/99 (H9N2), 56% against the H3N2 variant A/Indiana/08/11 (H3N2v), 11% against A/Hong Kong/G9/97 (H9N2), and 36% A/chicken/Hong Kong/SF4/01 (H6N1). None had titers >160 to A/Shanghai/2/13 (H7N9) or A/Netherlands/219/03 (H7N7). Thirty-six percent to 0% had neutralization titers to various H5N1 strains. Titers to H9, H6, and H5 HA-pseudoviruses correlated with each other, but not with H3N2v, suggesting group-specific cross-neutralization.