Occurrence of NA stalk deletions (SΔNA) among all reported avian influenza HA-NAa subtypes.

a: hemagglutinin-neuraminidase.b: Single number and the number behind slash denote the number of samples analyzed.c: Number before slash denotes the number of sequences with SΔNA.d: excluding sequences from HPAI H5N1 viruses.-: No isolate has been reported for the subtype in publically accessible database, GenBank.+:No samples were included due to unqualified sequences of the reported HA-NA subtype.*: Some cases of the subtype were reported in poultry outbreaks.