We also answer some common questions, such as whether the flu shot for one strain reduces the risk of contracting another strain and whether flu vaccines are reissued after the virus mutates during the course of the season. The seasonal flu viruses that humans face every winter season in the United States are caused by human influenza A. Influenza A viruses are categorized as either the hemagglutinin subtype or the neuraminidase subtype based on the proteins involved, and there are 18 distinct subtypes of hemagglutinin and 11 distinct subtypes of neuraminidase.
Influenza A is the primary cause of flu epidemics, and they constantly change and are difficult to predict. Meanwhile, influenza B viruses also affect humans and can be categorized by different strains and lineages. Influenza C viruses can cause mild respiratory illnesses but not epidemics, and modern flu vaccines do not protect against them.
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. Figure 2 — This image shows how influenza viruses are named. The name starts with the virus type, followed by the place the virus was isolated, followed by the virus strain number often a sample identifier , the year isolated, and finally, the virus subtype.
CDC follows an internationally accepted naming convention for influenza viruses. This convention was accepted by WHO in and published in February in the Bulletin of the World Health Organization, 58 4 see A revision of the system of nomenclature for influenza viruses: a WHO Memorandum pdf icon[ KB, 7 pages]external icon pdf icon external icon. The approach uses the following components:. Getting a flu vaccine can protect against these viruses as well as additional flu viruses that are antigenically similar to the viruses used to make the vaccine.
Seasonal flu vaccines do not protect against influenza C or D viruses or against zoonotic animal-origin flu viruses that can cause human infections, such as variant or avian flu viruses.
In addition, flu vaccines will NOT protect against infection and illness caused by other viruses that also can cause influenza-like symptoms. There are many other viruses besides influenza that can result in influenza-like illness ILI that spread during flu season.
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Facebook Twitter LinkedIn Syndicate. Types of Influenza Viruses. Minus Related Pages. Antigens on the internal proteins M1 and NP are type-specific and used to determine if a particular influenza virus is type A, B or C. Both M1and NP proteins of all members of each type exhibit cross reactivity. Hemagglutinin is a surface glycoprotein that binds to sialic acid residues on respiratory epithelial cell surface glycoproteins.
This interaction is necessary for attachment and fusion of viral and epithelial cell membranes. Neuraminidase digests sialic acid neuraminic acid on the surface of target cells, promoting entry of the virus into the cell.
Neuraminidase also facilitates penetration of the mucus layer in the respiratory tract. By late infection, almost all sialic acid has been removed from infected cell surface making it is easier for progeny virions to disseminate to other cells. N is the target of the antiviral drugs Relenza and Tamiflu. Influenza C viruses are somewhat different. They contain 7 RNA segments instead of eight. The major influenza C virus envelope glycoprotein is called HEF hemagglutinin-esterase-fusion because it has the functions of both the H and N.
A minor viral envelope protein is CM2, which functions as an ion channel. H and N exhibit more antigenic variation than the internal proteins and are the major determinants of Influenza A subtype and strain-specificity. Minor changes in the envelope glycoproteins, hemagglutinin and neuraminidase, are referred to as antigenic drift, and major changes are called antigenic shifts.
Antigenic drifts are associated with localized outbreaks, while antigenic shifts are associated with epidemics and pandemics of influenza A. Inefficient proofreading by influenza viral RNA polymerase results in a high incidence of transcription errors and amino acid substitutions in hemagglutinin or neuraminidase, allowing new variants to evade preexisting humoral immunity and cause influenza outbreaks.
An individual immune to the original strain is not immune to the drifted one. Wild aquatic birds are the natural hosts for all subtypes of influenza A virus.
Pigs also play an important role in the evolution of human pandemic strains because pig trachea contains receptors for both avian and human influenza viruses and pigs support the growth of both types of viruses. Genetic reassortment between avian and human virus may occur in pigs, leading to a novel strain. When a pig becomes infected with both human and avian viruses, the RNAs of both viruses are copied in the nucleus. When new virus particles are assembled at the cell membrane, some of the RNA segments may originate from either of the infecting virus.
New viruses that inherit RNA from both avian and human influenza are called reassortants. If this virus reassortant can infect humans, they will have little immunity to it, increasing the likelihood of an epidemic or pandemic.
The H1N1 pandemic that occurred in was due to reassortment of avian, human and swine influenza viruses. Reassortment can only occur between influenza viruses of the same type. Influenza B is much less likely to undergo antigenic shift, possibly because there is not an animal reservoir for this virus. Although 16 H and nine N virus subtypes occur in their natural reservoir of aquatic birds, only three hemagglutinin subtypes H1, H2, and H3 and two neuraminidase subtypes N1 and N2 have established stable lineages in humans and caused widespread human respiratory infection.
H1N1 and H3N2 cause seasonal epidemics today. Antigenic shifts have been responsible for several human pandemics. The extremely severe pandemic of and swine influenza or Spanish influenza was associated with the emergence of antigenic shifts in both hemagglutinin H1 and neuraminidase N1 of influenza A.
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