Pandemic Influenza

Influenza viruses cause annual epidemics and occasional pandemics that have claimed the lives of millions. The emergence of novel strains continues to challenge the public health and scientific communities. Recent outbreaks of highly pathogenic avian influenza A virus infections (including those of the H5N1 subtype) in poultry and in humans (through contact with infected birds), since 2003, have had important economic repercussions and have raised concerns that a new influenza pandemic involving H5N1 viruses is imminent. In the spring of 2009, a novel swine-origin H1N1 virus, whose antigenicity is quite different from those of human H1N1 strains, emerged in Mexico and readily transmitted and spread among humans, resulting in international outbreaks. This H1N1 virus was shown to be a triple reassortant comprising genes derived from avian, human, and swine viruses. On 11 June 2009, the World Health Organization declared that the infection caused by this new strain had reached pandemic proportion. Here, we review our current knowledge of pandemic influenza.