Influenza A viruses are zoonotic pathogens that continuously circulate and change in several animal hosts, including birds, pigs, horses, and humans. The viral pathogen causes infections with various consequences ranging from pandemics to seasonal flu. The emergence of novel virus strains that are capable of causing human epidemics or pandemics is a serious possibility [ 1 ]. The World Health Organization estimates that the global disease burden from influenza is around one billion infections, three million to five mil ion cases of severe disease, and between 300,000 and 500,000 deaths annually [ 2 ].

Influenza virus es contain 8 single-stranded RNA segments encoding 11 proteins. There are three types of influenza virus es: A, B, and C, with types A and B causing annual human epidemics. A key feature of the influenza virus is its error-prone polymerase, which results in an accumulation of genetic mutations that are selected for in hemagglutinin (HA) and to a lesser extent neuraminidase (NA)—the major surface glycoproteins of the virus. This antigenic drift of the HA protein renews our susceptibility to influenza viruses and is the basis for frequent updating of the composition of seasonal influenza vaccine s. Protection after natural infection is primarily mediated by HA-specific antibodies in serum and mucosa, with the presence of antibodies against NA, conserved influenza proteins, and T-cell responses correlating with reduced disease severity [ 2 ].

A novel virus can emerge in humans either through direct interspecies transmission or as a result of molecular exchanges between influenza virus es that already infect humans. Because the influenza virus genome is segmented, coinfection of a single host cell with two or more different influenza viruses can result in a reassortment (or shuffle) of their genetic material. The antigenic shift can lead to a pandemic if the resulting progeny virus contains an HA protein to which humans have no pre-existing immunity, if it has an efficient replication-competent set of internal genes, and if it can readily spread from human to human [ 2 ].

Vaccination is the primary strategy for the prevention and control of influenza. Seasonal influenza vaccine s are trivalent. Each dose is formulated to contain three viruses (or their HA proteins) representing the influenza A H3N2, influenza A H1N1, and influenza B strains considered to be the most likely to circulate in the upcoming influenza season [ 2 ]. Currently, most influenza vaccines are made from virus cultured in eggs, which is a severe production bottleneck during a serious threat of epidemic. There is an urgent need to develop a new efficacious process for influenza vaccine production which could be rapidly and cheaply manufactured [ 3 ]. The influenza virus has high mutation rate and a particular influenza vaccine usually confers protection for no more than a few years. Every year WHO predicts the strains of the virus that would be circulating in the following year and the vaccines are manufactured based on these data. The vaccine is formulated each season for a few specific flu strains but does not include all the strains active in the world during that season. A truly universal vaccine that provides lifelong protection against any strain of influenza with one or more vaccinations is certainly a goal that is worth pursuing [ 2 ].

References
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[1] Medina RA, García-Sastre A (2011) Influenza A viruses: new research developments. Nat Rev Microbiol 9:590–603

[2] Lambert LC, Fauci AS (2010) Influenza vac cines for the future. N Engl J Med 363: 2036–2044

[3] Thomas S, Luxon BA (2013) Vaccines based on structure-based design provide protection against infectious diseases. Expert Rev Vaccines 12:1301–1311