Strategies towards universal pandemic influenza vaccines.

Vaccination is considered to be the most effective and economical strategy against pandemic influenza. Vaccine development for multiple highly pathogenic avian influenza viruses, for example, H5N1, is hindered by antigenic drift, especially in the hemagglutinin (HA) sequence, as well as the antigenic shift. Growing efforts have been made to generate universal pandemic influenza vaccines. As mainly shown in animal trials, cross-clade and heterosubtypic protection by these universal vaccines are generally elicited by either a broad antigen-specific antibody response or influenza-specific CD4+ and CD8+ T-cell responses. Strain selection, HA engineering and broad neutralizing antigen determination are major strategies to achieve universal and specific antibody response, while studies on other factors including vectors, adjuvants and administration routes aim for enhanced T-cell responses against diverse influenza subtypes. Prospectively, cost-effective universal vaccines developed based on these combined technologies are promising solutions for broad protection against influenza.