Each of the following statements concerning the antigenicity of influenza A virus is correct EXCEPT:

  •  Antigenic shifts affect both the hemagglutinin and the neuraminidase.
  •  The worldwide epidemics caused by influenza A virus are due to antigenic shifts.
  •  The protein involved in antigenic drift is primarily the internal ribonucleoprotein.
  •  Antigenic shifts, which represent major changes in antigenicity, occur infrequently and are due to the reassortment of segments of the viral genome.