Influenza, commonly known as the “flu”, is an acute infection of the respiratory tract caused by influenza viruses. There are three types of seasonal influenza viruses – A, B and C. Influenza A viruses are further categorised into subtypes, and influenza B into lineages. The 2009 pandemic influenza A(H1N1) virus (hereafter referred to as influenza A(H1N1)pdm09) which appeared for the first time in 2009 causing a global influenza pandemic, is now a seasonal influenza virus that co-circulates with other seasonal viruses (namely influenza A(H3N2) and influenza B viruses). Influenza viruses are genetically dynamic and evolve in unpredictable ways.
Influenza viruses are further classified based upon antigenic properties. Humoral immunity to influenza viruses is generally thought to be strainspecific and acquired through infection and/or vaccination. Seasonal influenza epidemics can be caused by evolving virus strains that are antigenically distinct from previously circulating virus strains to which a population has some immunity; this is known as antigenic drift. Uncommonly, a completely new strain of influenza will emerge to which there is little or no existing immunity, this is known as antigenic shift and such novel strains can give rise to influenza pandemics.
Influenza viruses are spread from person-to-person. They can be transmitted by exposure to infectious droplets expelled by coughing or sneezing that are then inhaled by others, or can contaminate hands or other surfaces. The typical incubation period for influenza is 1-4 days. Most persons ill with influenza shed virus (i.e. may be infectious) from a few days before symptoms begin through 5-7 days after illness onset. However, very young children can be infectious for >10 days after illness onset; adults with severe disease (e.g. viral pneumonia) may also shed virus for >10 days, and severely immunocompromised persons can shed virus for even longer. Children have the highest rates of seasonal influenza infection and illness in this group can amplify viral transmission in the community.
