There are four types of influenza viruses: A, B, C, and D. Influenza A and B viruses cause seasonal epidemics of disease in people (known as flu season) almost every winter in the United States. Influenza A viruses are the only influenza viruses known to cause flu pandemics (i.e., global epidemics of flu disease). A pandemic can occur when a new and different influenza A virus emerges that infects people, has the ability to spread efficiently among people, and against which people have little or no immunity. Influenza C virus infections generally cause mild illness and are not thought to cause human epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively). 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. Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information. Current subtypes of influenza A viruses that routinely circulate in people include A(H1N1) and A(H3N2). 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.
Clades and sub-clades can be alternatively called “groups” and “sub-groups,” respectively. 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. (See the for more information). 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). Dividing viruses into clades and subclades helps flu experts track the proportion of viruses from different clades in circulation.
Note that clades and sub-clades that are genetically different from others are not necessarily . This is best understood by first introducing the concepts of “antigens” and “antigenic properties”. As previously described, flu viruses have hemagglutinin (H) and neuraminidase (N) surface proteins. These proteins act as antigens. Antigens are molecular structures on the surface of viruses that are recognized by the immune system and can trigger an immune response (such as antibody production). The antigenic properties are a reflection of the antibody or immune response triggered by the antigens on a particular virus. When two flu viruses are antigenically different, this means that a host’s immune response (antibodies) elicited by infection or vaccination with one of the viruses will not as easily recognize and neutralize the other virus. Therefore, for antigenically different viruses, immunity developed against one of the viruses will not necessarily protect against the other virus as well.
Conversely, when two flu viruses are antigenically similar, a host’s immune response (antibodies) elicited by infection or vaccination with one of the viruses will recognize and neutralize the other virus, thereby protecting against the other virus.
Currently circulating influenza A(H1N1) viruses are related to the pandemic 2009 H1N1 virus that emerged in the spring of 2009 and caused a flu pandemic. These viruses, scientifically called the “A(H1N1)pdm09 virus,” and more generally called “2009 H1N1,” have continued to circulate seasonally since then and have undergone genetic changes and changes to their (i.e., the properties of the virus that affect immunity).
Influenza A(H3N2) viruses also change both genetically and antigenically. Influenza A(H3N2) viruses have formed many separate, genetically different clades in recent years that continue to co-circulate.
Influenza B viruses are not divided into subtypes, but instead are further classified into two lineages: B/Yamagata and B/Victoria. Similar to influenza A viruses, influenza B viruses can then be further classified into specific clades and sub-clades. Influenza B viruses generally change more slowly in terms of their genetic and antigenic properties than influenza A viruses, especially influenza A(H3N2) viruses. Influenza surveillance data from recent years shows co-circulation of influenza B viruses from both lineages in the United States and around the world. However, the proportion of influenza B viruses from each lineage that circulate can vary by geographic location and by season. In recent years, flu B/Yamagata viruses have circulated much less frequently in comparison to flu B/Victoria viruses globally.
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There are four types of influenza viruses: A, B, C, and D. Influenza A and B viruses cause seasonal epidemics of disease in people (known as flu season) almost every winter in the United States. Influenza A viruses are the only influenza viruses known to cause flu pandemics (i.e., global epidemics of flu disease). A pandemic can occur when a new and different influenza A virus emerges that infects people, has the ability to spread efficiently among people, and against which people have little or no immunity. Influenza C virus infections generally cause mild illness and are not thought to cause human epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively). 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. Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information. Current subtypes of influenza A viruses that routinely circulate in people include A(H1N1) and A(H3N2). 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.
Clades and sub-clades can be alternatively called “groups” and “sub-groups,” respectively. 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. (See the for more information). 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). Dividing viruses into clades and subclades helps flu experts track the proportion of viruses from different clades in circulation.
Note that clades and sub-clades that are genetically different from others are not necessarily . This is best understood by first introducing the concepts of “antigens” and “antigenic properties”. As previously described, flu viruses have hemagglutinin (H) and neuraminidase (N) surface proteins. These proteins act as antigens. Antigens are molecular structures on the surface of viruses that are recognized by the immune system and can trigger an immune response (such as antibody production). The antigenic properties are a reflection of the antibody or immune response triggered by the antigens on a particular virus. When two flu viruses are antigenically different, this means that a host’s immune response (antibodies) elicited by infection or vaccination with one of the viruses will not as easily recognize and neutralize the other virus. Therefore, for antigenically different viruses, immunity developed against one of the viruses will not necessarily protect against the other virus as well.
Conversely, when two flu viruses are antigenically similar, a host’s immune response (antibodies) elicited by infection or vaccination with one of the viruses will recognize and neutralize the other virus, thereby protecting against the other virus.
Currently circulating influenza A(H1N1) viruses are related to the pandemic 2009 H1N1 virus that emerged in the spring of 2009 and caused a flu pandemic. These viruses, scientifically called the “A(H1N1)pdm09 virus,” and more generally called “2009 H1N1,” have continued to circulate seasonally since then and have undergone genetic changes and changes to their (i.e., the properties of the virus that affect immunity).
Influenza A(H3N2) viruses also change both genetically and antigenically. Influenza A(H3N2) viruses have formed many separate, genetically different clades in recent years that continue to co-circulate.
Influenza B viruses are not divided into subtypes, but instead are further classified into two lineages: B/Yamagata and B/Victoria. Similar to influenza A viruses, influenza B viruses can then be further classified into specific clades and sub-clades. Influenza B viruses generally change more slowly in terms of their genetic and antigenic properties than influenza A viruses, especially influenza A(H3N2) viruses. Influenza surveillance data from recent years shows co-circulation of influenza B viruses from both lineages in the United States and around the world. However, the proportion of influenza B viruses from each lineage that circulate can vary by geographic location and by season. In recent years, flu B/Yamagata viruses have circulated much less frequently in comparison to flu B/Victoria viruses globally.
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