Multiple factors could play a role in defining the true correlates of protection to influenza infection and disease and many of these factors are yet to be clearly defined. In our own influenza challenge study, protocol 12-I-0077, we have clearly seen evidence that not everyone with a low HAI titer is susceptible to influenza, and that there must be other factors protecting certain individuals. There are many examples like this that demonstrate that there may be much more to immune protection to influenza than just anti-HA antibodies.