Epidemiology

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Prévention de l’InfluenzaPublished by: Groupe de travail Développement de recommandations de première ligneLast published: 2018Preventie van influenzaPublished by: Werkgroep Ontwikkeling Richtlijnen Eerste Lijn (Worel)Last published: 2018

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. Additionally, incidence varies each year, with antigenic change in the viruses meaning that there is no incremental protection from previous immunisations. Overall, influenza infection 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] It is estimated that influenza resulted in 120,000-710,000 hospitalisations and 6,300-52,000 deaths annually between 2010 and 2024 in the US.[6]

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.[7]

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.[8] In the 2017-2018 season, influenza activity in the US was also reported to have reached its highest level since the 2009 H1N1 pandemic.[6]​​

In the 2020/21 and 2021/22 season, influenza activity levels were extremely low as a result of measures to prevent coronavirus disease 2019 (COVID-19) (such as handwashing, mask-wearing, physical distancing, and restricted travel). In the 2022/23 season, there was a resurgence of influenza activity as social contact returned to normal.[6][9][10]​​

In 2023 a single confirmed human case of influenza A(H1N2)v, which is similar to flu viruses circulating in pigs in the UK, was detected​​​​​​. This was the first case of this strain of flu in a human in the UK.[11]

In 2005, 1918 influenza A (H1N1) virus was successfully reconstructed, enabling a better 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 antiviral 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.[12]​​

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