Hemagglutinin (HA) and neuraminidase (NA) are major glycoproteins expressed on 
the surface of influenza virus. They have complementary and antagonistic 
functions that facilitate in the life cycle of the virus. The functional 
equilibrium generated between HA and NA can impact the evolution and adaptation 
of influenza virus strains within the human reservoir. This functional 
equilibrium is referred to as the "HA-NA balance". An imbalanced HA-NA can 
restrict the multiplication and transmission capacity of influenza viruses. 
Moreover, this equilibrium is likely a limiting factor against species crossover 
for the virus. In light of such considerations, the HA-NA balance should be 
precisely studied to gain a better understanding of the emergence of pandemic 
and seasonal influenza virus strains. This review describes the concept of the 
HA-NA balance, the methods used to study it, plus a discussion of the HA-NA 
balance in the evolution of the pandemic influenza A H1N1 strains that plagued 
the world in 1918 and 2009.