Influenza Virus Evolution, Host Factors and the Assessment of Influenza Vaccine Effectiveness

Influenza virus vaccines are the main prophylactic strategy for reducing the burden of influenza morbidity and mortality. Nevertheless the currently available influenza vaccines induce a narrow and strain specific immunity and their protective effect is limited by the continuous evolution of influenza viruses associated with rapidly evolving mutations in key antigenic sites of the hemagglutinin surface protein.

Besides viral factors, also host and environmental factors considerably influence the protective effect of influenza vaccines considerably. Assessment of vaccine effectiveness (VE) using the test-negative case-control design has revolutionized VE monitoring and has contributed to a better understanding of suboptimal VE of seasonal influenza vaccines.

This methodology first described for the 2004/05 influenza season in Canada is now the preferred observational study design to reliably calculate the effectiveness of seasonal influenza vaccines against medically attended influenza virus infections.

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