An HI titer of 40 is associated with 50% protection of patients from seasonal influenza illness.8, 9, 10 Evidence for the contribution of NI antibody to the protection against seasonal influenza, independent of the effect of HA antibody, has been suggested by patterns of infection during the 1968 pandemic and more recently using multivariable regression analyses.11, 12, 13 Given the relative high NI titers observed here in comparison with HI titers, natural infection response to NA might have a longer duration of protection even when HA drifts, but NA does not. Here, XK is linked to infection.