Neuraminidase inhibition (NI) activity of IgG1, monomeric (M) or polymeric (P) IgA1, and monomeric (M) or polymeric (P) IgA2m2 derived from antibody clones F11 (A–D), FI6 (E-H), and B12 (I–L), as well as Fab-deficient (ΔFab) IgA antibodies (M–P), against virus strains A/New Caledonia/20/1999 (H1N1, NC20) (A, B, E, F, I, J, M, and N) and A/Victoria/210/2009 (H3N2, Vic210) (C, D, G, H, K, L, O, and P). NI activity is presented as inhibition curves, with each point and error bar representing the mean ± SD of three technical replicates. Y-axis values represent percentage inhibition of neuraminidase activity. The OD values of wells incubated without antibodies were normalized to y = 100, and the OD values of wells incubated without virus were normalized to y = 0. X-axis values represent the concentration of antibody added to each well. Zanamivir, a neuraminidase inhibitor, was used as a positive control. *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001, comparing monomers with polymers (two-way ANOVA).