DNA viruses, therefore, do not change, or mutate, much. Mistakes in copying RNA happen frequently, and the host cell does not correct these mistakes. RNA virus mutations are frequent and can have important consequences for their hosts. Influenza viruses are simple entities belonging to one of three types: A, B, or C. They consist of no more than seven or eight RNA segments enclosed within an envelope of proteins.
Influenza viruses can evolve in a gradual way through mutations in the genes that relate to the viral surface proteins hemagglutinin and neuraminidase HA and NA in shorthand. In such a case, antibodies produced by previous infection with the ancestor strain cannot effectively fight the mutated virus, and disease results. Hemagglutinin and neuraminidase lend their first initials to flu subtypes. For example, the influenza pandemic was caused by an influenza A H1N1 virus.
Antigenic drift is one reason that new flu vaccines often need to be created for each flu season. Scientists try to predict which changes are likely to occur to currently circulating flu viruses. They create a vaccine designed to fight the predicted virus.
Sometimes the prediction is accurate, and the flu vaccine is effective. Antigenic shift is a process by which two or more different types of influenza A combine to form a virus radically different from the ancestor strains.
The virus that results has a new HA or NA subtype. Antigenic shift may result in global disease spread, or pandemic, because humans will have few or no antibodies to block infection. However, if the new influenza A subtype does not easily pass from person to person, the disease outbreak will be limited.
Antigenic shift occurs in two ways. First, antigenic shift can occur through genetic recombination, or reassortment, when two or more different influenza A viruses infect the same host cell and combine their genetic material.
Influenza A viruses can infect birds, pigs, and humans, and major antigenic shifts can occur when these virus types combine.
For example, a pig flu virus and a human flu virus could combine in a bird, resulting in a radically different flu type. If the virus infects humans and is efficiently transmitted among them, a pandemic may occur. Second, an influenza A virus can jump from one type of organism, usually a bird, to another type of organism, such as a human, without undergoing major genetic change.
If the virus mutates in the human host so that it is easily spread among people, a pandemic may result. In all cases, antigenic shift produces a virus with a new HA or NA subtype to which humans have no, or very few, preexisting antibodies.
Once scientists are able to identify the new subtype, a vaccine can generally be created that will provide protection from the virus. Why does antigenic shift occur only with influenza A, and not influenza B and C? Influenza A is the only influenza type that can infect a wide variety of animals: humans, waterfowl, other birds, pigs, dogs, and horses.
Recombination possibilities, therefore, are very low or nonexistent with influenza B and C. A pandemic had the potential to occur in the bird flu outbreaks in in Asia.
An H5N1 influenza A virus spread from infected birds to humans, resulting in serious human disease. But the virus has not evolved to be easily spread among humans, and an H5N1 pandemic has not occurred. First, it reproduces much more rapidly than most other entities. It can produce billions of copies of itself each day. As it makes rapid-fire copies of itself, it commonly makes errors, which translate into mutations in its genetic code.
This happens when a host cell is infected with two different variations of HIV. Elements of the two viruses may combine to result in a new virus that is a unique combination of the two parents.
The rapid rate of HIV evolution has important consequences. Additionally, targeting a vaccine to a rapidly changing virus is challenging. To date, researchers have developed several candidate HIV vaccines, but none has performed well enough in clinical trials to warrant licensure. Burke, D. Recombination in HIV: An important viral evolutionary strategy.
Emerging Infectious Diseases. How the flu virus can change: Shift and drift. Types of influenza viruses. University of California Museum of Paleontology. Viruses change all the time. That's because they copy themselves to reproduce. Think of our cells as having their own xerox machines, which the virus takes over for its own purposes. When a virus makes a copy, sometimes a random change can occur in the copy's DNA -- the double spiral-shaped molecule which acts as a manual telling our bodies how to develop and function.
If enough of these changes happen over time, a new variation or strain of a virus can emerge. Mutations happen in two main ways. In the first, small copying errors in the virus lead to changes in the virus' surface proteins , which sit on the outside of the virus.
These proteins look to attach to your cells much like boats seeking to tie to a dock. These changes result in more closely related virus variations, like the new Covid variants. In the other way, two variations infect the same cell in a person's body and combine to form a new or "novel" virus.
This often happens when a variation that only infects animals comes into contact with a human variation. This version of mutation could have created the novel coronavirus which causes Covid We become concerned when mutations make the virus more deadly, more contagious or both. Some mutations in the S. African variant might also make it able to evade the first set of antibodies the immune system uses against it.
The UK variant, called Alpha, B. Some of the mutations increase infectivity, others increase the number of infected cells by helping the virus avoid the immune system.
The South African variant, called Beta, B. The P1 variant from Brazil, called Gamma, is similar to South African variant with one extra mutation. The Delta variant, B. In the projection models and real-life experience so far, both the U.
It is also possible that these strains will become the predominant strains all over the globe. So far, it does not look like that there is any difference in the disease severity caused by these strains vs.
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