2020–2023 H5N8 outbreak

In the early 2020s, an ongoing outbreak of avian influenza subtype H5N8 has been occurring at poultry farms and among wild bird populations in several countries and continents, leading to the subsequent cullings of millions of birds to prevent a pandemic similar to that of the H5N1 outbreak in 2008. The first case of human transmission of avian flu, also known as bird flu, was reported by Russian authorities in February 2021, as several poultry farm workers tested positive for the virus.

The H5N8 outbreak was marked by a H5 hemagglutinin (HA) of the 2.3.4.4b lineage. A virus with this lineage of HA gene, either H5N8 or H5N6, would later give rise to a H5N1 virus after reassortment (genetic "swapping") with an influenza virus with N1 in nature, the culprit for the 2020–2025 H5N1 outbreak. The 2.3.4.4 lineage originated in China in 2008 and has dominated the global scene since 2014. The 2.3.4.4b sub-lineage became highly noteworthy among birds in 2016. The initial outbreaks in 2016–2020 were mostly due to H5N8, after which the H5N1 form became more significant. It also lead to some smaller outbreaks with other types of the neuroamidase (N) gene.

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