Drs. Jose Victor Zambrana and Gordon report that 23% of RT-PCR-confirmed A(H3N2) infections were missed by the gold-standard HAI assay, while 62% of HAI non-responders mounted alternate antibody responses. They found high pre-existing immunity and the absence of influenza-like illness were independently associated with failure to mount an HAI response. They support incorporating broader serologic measures to provide a more comprehensive understanding of host immunity beyond traditional HAI endpoints, with important implications for influenza surveillance and vaccines.
Key Terms: hemagglutination inhibition (HAI); correlates of protection; influenza A/H3N2
