Background.  During the 2009 pandemic of influenza A virus subtype H1N1 (A[H1N1]pdm09) infection, older individuals were partially protected from severe disease. It is not known whether preexisting antibodies with effector functions such as antibody-dependent cellular cytotoxicity (ADCC) contributed to the immunity observed.

Methods.  We tested serum specimens obtained from 182 individuals aged 1–72 years that were collected either immediately before or after the A(H1N1)pdm09 pandemic for ADCC antibodies to the A(H1N1)pdm09 hemagglutinin (HA) protein.

Results.  A(H1N1)pdm09 HA-specific ADCC antibodies were detected in almost all individuals aged >45 years (28/31 subjects) before the 2009 A(H1N1) pandemic. Conversely, only approximately half of the individuals aged 1–14 years (11/31) and 15–45 years (17/31) had cross-reactive ADCC antibodies before the 2009 A(H1N1) pandemic. The A(H1N1)pdm09-specific ADCC antibodies were able to efficiently mediate the killing of influenza virus–infected respiratory epithelial cells. Further, subjects >45 years of age had higher ADCC titers to a range of seasonal H1N1 HA proteins, including from the 1918 virus, compared with younger individuals.

Conclusions.  ADCC antibodies may have contributed to the protection exhibited in older individuals during the 2009 A(H1N1) pandemic. This work has significant implications for improved vaccination strategies for future influenza pandemics.