H2N2 influenza viruses emerged in the 1950s replacing the then circulating H1N1 human influenza virus. The first cases occurred in China in 1956, and disease became widespread in 1956-1957, resulting in the "Asian Influenza" pandemic that was responsible for between 1 and 4 million deaths worldwide. H2N2 viruses have not circulated since 1968, when they were replaced by H3N2 influenza viruses and the resurgence of H1N1 viruses. For this reason, a large proportion of the population is now susceptible to infection with H2N2 influenza. If this subtype re-emerges, it could potentially cause the next pandemic. This vaccine, therefore, is an important priority in the development of vaccines against potential pandemic influenza strains.