Influenza A/H1N1 in 2009: a pandemic in evolution.

Influenza has demonstrated its capacity to surprise with the emergence of a novel influenza A/H1N1 virus in North America. Air travel has a facilitated rapid inter continental spread of the virus, threatening the next pandemic. Current seasonal influenza A/H1N1, A/H3N2 and B viruses infect approximately 5–15% of the world’s population each year, and are responsible for an estimated 500,000 excess deaths annually. Seasonal morbidity and mortality are highest during A/H3N2 outbreaks, principally in young children, the elderly and those with chronic medical conditions. Effective annual immunization programs of seasonal vaccination generally focus on these three at-risk groups. Increased seasonal vaccine demand since the last pandemic has sustained an increase in vaccine manufacturing capacity that will be vital to a pandemic response. Pandemics of influenza occur unpredictably when a novel virus with the capacity for person-to-person transmission emerges to which there is limited immunity in the population. During pandemics, agespecific attack rates may differ from those observed in seasonal outbreaks, with a greater impact on young adults. Reasons for the decline and replacement of existing inf luenza subtypes are uncertain. Experience from the 20th Century suggests that novel strains emerge from the animal reservoir of influenza A viruses following genetic reassortment between human and nonhuman strains. The pandemic H1N1 virus in 1918, H2N2 in 1957 and H3N2 in 1968 emerged to establish human lineages. Human–avian reassortant viruses caused the pandemics of 1957 and 1968, when avian viruses contributed the hemagglutinin gene to the pandemic strain. Genomic sequencing of the reconstructed 1918 pandemic H1N1 virus indicates its origin as an adapted avian virus without reassortment.