Seasonal influenza epidemics represent a substantial public health burden, causing significant morbidity and mortality. Influenza in humans can be caused by influenza type A and type B viruses, and although influenza A is responsible for the majority of seasonal influenza infections, influenza B disease is common in children and young adults, and causes seasonal epidemics every 2-4 years. Influenza strains circulating during a seasonal epidemic may be influenza type A strains A/H1N1 and A/H3N2, strains of influenza B lineages B/Victoria and B/Yamagata, or any combination of these. The morbidity and mortality burden of influenza infections means that public health agencies worldwide recommend vaccination to try and protect against seasonal epidemics. The World Health Organization (WHO) and, in the United States of America (USA), the Food and Drug Administration (FDA), recommend trivalent seasonal influenza vaccines containing the two main influenza type A strains and, due to its lesser but still important prevalence, one influenza type B strain. There is little or no cross-reactive protection between the influenza B lineages: this means that good protection against the circulating virus relies on correctly predicting the prevalent influenza B lineage in any season. This has proved to be reliant on chance, and little or no protection has been provided in the USA by the trivalent vaccines against the circulating influenza B virus in 5 of the 10 seasons between 2001 and 2010. There is, therefore, a clear need for a quadrivalent influenza vaccine, containing influenza A/H1N1, A/H3N2, and B/Victoria and B/Yamagata lineage strains, to improve protection against influenza B virus and reduce the morbidity of influenza B infection.
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