The recurrent problem in human influenza is the pandemic increase in mortality and social dislocation which occurs every 10-15 years whenever a new subtype of type A virus emerges, with antigens which cannot be neutralized by antibodies induced in the population by infection or vaccination with any of the foregoing epidemic strains.
There is an urgent need to interpret these past antigenic shifts so that the constitution of the next pandemic strain can be predicted, allowing effective specific vaccines to be made in advance.
The pandemic viruses are unlikely to be direct mutants from one to another. Epidemiological evidence of their cyclic recurrence in mankind at 70 year intervals suggests the role of animal reservoirs of influenza viruses. Man may be infected directly, or after genetic recombination between an animal virus with novel antigens and a human strain with the necessary virulence for man.