The failure to mount an antibody response following viral infection or seroconversion failure is a largely underappreciated and poorly understood phenomenon. Here, we identified immunologic markers associated with robust antibody responses after influenza virus infection in two independent human cohorts, SHIVERS and FLU09, based in Auckland, New Zealand and Memphis, Tennessee, USA, respectively. In the SHIVERS cohort, seroconversion significantly associates with (1) hospitalization, (2) greater numbers of proliferating, activated CD4+ T cells, but not CD8+ T cells, in the periphery during the acute phase of illness, and (3) fewer inflammatory monocytes (CD14hiCD16+) by convalescence. In the FLU09 cohort, fewer CD14hiCD16+ monocytes during early illness in the nasal mucosa were also associated with the generation of influenza-specific mucosal immunoglobulin A (IgA) and IgG antibodies. Our study demonstrates that seroconversion failure after infection is a definable immunological phenomenon, associated with quantifiable cellular markers that can be used to improve diagnostics, vaccine efficacy, and epidemiologic efforts. Display omitted Post-infection seroconversion is associated with severity of influenza virus infectionSeroconverters have early proliferation and activation of CD4+ T cellsCD8+ T cells are unaffectedCD14hiCD16+ monocytes in the blood and nasal mucosa is associated with antibody response Wong et al. show that antibody responsiveness after influenza virus infection is associated with CD4+ T cells and CD14hiCD16+ monocytes. CD14hiCD16+ monocytes are also important in the mucosal antibody response. This demonstrates that seroconversion failure after infection is a definable immunological phenomenon, an important consideration for diagnostics and epidemiological studies.