The 1957 flu pandemic was caused by influenza H2N2 virus, to which few people had previous exposure. A vaccine was rapidly developed against H2N2, though later assessment showed that more vaccine than usual was needed to produce immunity. H2N2 no longer circulates in humans but is carried by animals. Because of the pandemic potential of H2 viruses, researchers are developing putative H2 vaccines as part of pre-pandemic vaccine planning.