Antigenic shift can happen in three ways: Antigenic Shift 1 A duck or other aquatic bird passes a bird strain of influenza A to an intermediate host such as a chicken or pig. A person passes a human strain of influenza A to the same chicken or pig. When the viruses infect the same cell, the genes from the bird strain mix with genes from the human strain to yield a new strain.
The new strain can spread from the intermediate host to humans. Antigenic Shift 2 Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to humans. Antigenic Shift 3 Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to an intermediate animal host and then to humans. Abstract Numerous modern technological and scientific advances have changed the vaccine industry.
Publication types Review. Substances Influenza Vaccines. CDC and other public health laboratories around the world have been sequencing the gene segments of influenza viruses since the s. The sequences deposited into these databases allow CDC and other researchers to compare the genes of currently circulating influenza viruses with the genes of older influenza viruses and those used in vaccines.
This process of comparing genetic sequences is called genetic characterization. CDC uses genetic characterization for several reasons:. Each sequence from a specific influenza virus has its own branch on the tree. Viruses are grouped by comparing changes in nucleotides within the gene. Viruses which share a common ancestor can also be described as belonging to the same clade.
The degree of genetic difference number of nucleotide differences between viruses is represented by the length of the horizontal lines branches in the phylogenetic tree. The further apart viruses are on the horizontal axis of a phylogenetic tree, the more genetically different the viruses are to one another.
Phylogenetic trees of influenza viruses will usually display how similar sequences of the nucleotides for hemagglutinin HA genes of the vaccine virus and circulating viruses are to each other. As part of this process, CDC compares the new virus sequence with the other virus sequences and looks for differences among them.
CDC then uses a phylogenetic tree to visually represent how genetically similar the A H3N2 viruses are to each other. In Figure 1, virus b is more genetically similar to virus c than d. Viruses b and c share a common ancestor and the total length of the horizontal branches is short. In this process mutations slowly accumulate in the viral genome.
This leads to an increasing diversity within the virus. In particular mutations that affect the surface antigens of the virus are important. Some of these mutations will give the virus a survival advantage that allows it to cause epidemics AKA the seasonal flu outbreaks that are mostly restricted to individual countries.
However, since the new version of the virus still resembles strains from previous epidemics part of the population remains immune. There are 3 types of influenza virus; types A, B, and C. Types A and B cause annual epidemics while type C not nearly as common and often causes mild symptoms. Type A is found in many species and therefore antigenic shift is possible and pandemics are more likely.
Type B is only found in humans and therefore slower antigenic drift changes lead to pandemics less often. Hemaglutinin is important for the virus to be able to bind to target cells and insert its viral genome. Neuraminidase is important during the process of the virus releasing offspring from the infected cell. There are many subtypes of hemagglutinin and neuraminidase, but only H and N are found commonly in humans. You have likely heard of subtypes of the Influenza A virus such as H1N1 which includes hemagglutinin 1 and neuraminidase 1.
This is the subtype of the virus that caused the Spanish Flu pandemic in and the Swine Flu pandemic in The body creates antibodies against Hemagluttinin.
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