The incidence of seasonal influenza infection is difficult to determine, as not everyone with influenza will seek medical attention, and not everyone with influenza-like illness will have influenza infection; diagnosis may be made clinically without laboratory confirmation. Additionally, incidence varies each year, with antigenic change in the viruses meaning that there is no incremental protection from previous immunisations. Overall, it is estimated to affect 20% of children and 5% of adults worldwide each year.[5] Studies in children report an average annual incidence of 4.6% over a 5-year period in children up to 19 years of age. Over a 25-year period in the US, the incidence was 9.5% of children <5 years of age.[5] In the northern hemisphere, seasonal influenza activity peaks between late December and early March, and in the southern hemisphere it peaks between May and September.

There have been four influenza pandemics since 1918, with the most recent being the Influenza A (H1N1) 'swine flu' epidemic in April 2009. In 1957 and 1968, both pandemics were as a result of novel strains of both human and avian influenza. However, the pandemic of 2009 was as a result of a novel gene rearrangement of human, avian, and swine influenza.[6]

In 2017, a fast-mutating strain of influenza A (H3N2) was reported in Australia, which saw the highest number of cases since the 2009 pandemic.[7] In the 2017-2018 season, influenza activity in the US is also reported to have reached its highest level since the 2009 H1N1 pandemic. CDC: FluView - weekly influenza surveillance report external link opens in a new window

In 2005, researchers at the Centers for Disease Control and Prevention (CDC) successfully reconstructed the 1918 Influenza A (H1N1) virus, enabling better a understanding of its virulence. It is highly unlikely that the 1918 virus would re-emerge from a natural source and, even if it did, residual immunity means it would no longer be considered a novel strain. If it was ever isolated outside of a laboratory, current treatments (such as the antivirals rimantadine and oseltamivir) would likely be effective and there would be the potential for vaccines, as those containing the 1918 haemagglutinin protein have been shown to be protective in mice.[8]

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