Dividing viruses into clades and subclades allows flu experts to track the proportion of viruses from different clades in circulation. Clades and subclades are shown on phylogenetic trees as groups of viruses that usually have similar genetic changes (i.e., nucleotide or amino acid changes) and have a single common ancestor represented as a node in the tree (see Figure 1). (See the Genome Sequencing and Genetic Characterization page for more information).
An influenza clade or group is a further subdivision of influenza viruses (beyond subtypes or lineages) based on the similarity of their HA gene sequences. Influenza viruses whose HA genes share the same genetic changes and who also share a common ancestor (node) are grouped into specific “clades” and “sub clades.”Ĭlades and sub-clades can be alternatively called “groups” and “sub-groups,” respectively. The further apart viruses are on the horizontal axis of a phylogenetic tree, the more genetically different the viruses are to one another. The degree of genetic difference between viruses is represented by the length of the horizontal lines (branches) in the phylogenetic tree. Each sequence from a specific influenza virus has its own branch on the tree. Phylogenetic trees of influenza viruses will usually display how similar the viruses’ hemagglutinin (H or HA) or neuraminidase (N or NA) genes are to one another. Phylogenetic trees show how closely ‘related’ individual viruses are to one another. Influenza A subtypes can be further broken down into different genetic “clades” and “sub-clades.” See the “Influenza Viruses” graphic below for a visual depiction of these classifications.įigure 1 – This is a picture of a phylogenetic tree. Current subtypes of influenza A viruses that routinely circulate in people include: A(H1N1) and A(H3N2). Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information.
While more than 130 influenza A subtype combinations have been identified in nature, primarily from wild birds, there are potentially many more influenza A subtype combinations given the propensity for virus “reassortment.” Reassortment is a process by which influenza viruses swap gene segments. There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively). Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people. Influenza C virus infections generally cause mild illness and are not thought to cause human epidemics.
A pandemic can occur when a new and different influenza A virus emerges that both infects people and has the ability to spread efficiently among people. Influenza A viruses are the only influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease. Human influenza A and B viruses cause seasonal epidemics of disease (known as flu season) almost every winter in the United States. B.V.There are four types of influenza viruses: A, B, C and D.
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