XK and infection: The heterologous neuraminidase (NA) vaccine strategy employs an inactivated AIV containing a similar subtype of hemagglutinin (HA) but a different subtype of NA compared to those of the outbreak strain (13), which allows screening for infection by determining heterologous anti-NA antibodies using an indirect immunofluorescence assay (13) or a conventional neuraminidase inhibition (NI) test (14).