The Question

In April 2009, a novel human influenza A (H1N1) virus was identified. This virus rapidly spread around the globe causing significant morbidity and mortality in 2009/10. This virus was of swine origin and contained a novel combination of gene segments not previously reported in a human influenza virus isolate. Except for the elderly, the vast majority of individuals around the world did not have protective immunity against the virus and were thus susceptible to infection. This relatively low immunological pressure has presumably contributed to the fact that there has been only limited antigenic change in the virus.