Abstract
The hemagglutination inhibition (HAI) antibody titer is considered the primary immune correlate of protection for influenza. However, recent studies have highlighted the limitations on the use of the HAI titer as a correlate in at-risk populations such as children and older adults. In addition to the neutralization of cell-free virus by antibodies to hemagglutinin and interference of virus release from infected cells by antibodies to neuraminidase, influenza virus-specific antibodies specifically can bind to infected cells and lyse virus-infected cells through the activation of complement or natural killer (NK) cells, via antibody-dependent cellular cytotoxicity (ADCC) or complement-dependent lysis (CDL). We evaluated preexisting HAI, CDL, and ADCC antibodies in young children enrolled in a prospective cohort study of dengue during the epidemic with influenza A(H1N1)pdm09 virus to determine associations between preexisting antibodies and the occurrence of clinical or subclinical influenza virus infection. Though both preexisting HAI and CDL antibodies were associated with protection against clinical influenza, our data suggested that CDL was not a better correlate than HAI. We found that ADCC antibodies behaved differently from HAI and CDL antibodies. Unlike HAI and CDL antibodies, preexisting ADCC antibodies did not correlate with protection against clinical influenza. In fact, ADCC antibodies were detected more frequently in the clinical influenza group than the subclinical group. In addition, in contrast to HAI and CDL antibodies, HAI and the ADCC antibodies titers did not correlate. We also found that ADCC, but not CDL or HAI antibodies, positively correlated with the ages of the children.