Neuraminidase Inhibition Enzyme-Linked Lectin Assay (NI-ELLA)

The NI‑ELLA is the most widely used semi‑quantitative assay for measuring antibodies against influenza neuraminidase (NA). The assay is based on the enzymatic activity of NA to cleave sialic acid residues from fetuin immobilized on microtiter plates, exposing galactose residues that are detected by peanut agglutinin (PNA) conjugated to horseradish peroxidase (HRP). Following substrate addition, a colorimetric reaction proportional to NA activity is measured as optical density (OD). In the presence of anti‑NA antibodies, NA activity is inhibited, resulting in reduced signal intensity. The NI antibody titer is defined as the reciprocal of the highest serum dilution that inhibits NA activity to a predefined threshold level.

VisMederi developed the NI-ELLA assay using lentiviral pseudotypes that incorporate the neuraminidase (NA) of interest together with a mismatched, avian-derived hemagglutinin (typically H11) that does not circulate in humans. This design minimizes non-specific inhibition caused by HA-binding antibodies that may be present in serum samples while keeping optimal assay performance.