This process explains the occurrence of seasonal influenza epidemics that may differ in severity and age groups affected.
It also accounts for vaccine mismatch when one of the strains selected for the vaccine does not optimally match the circulating strains. As a result, annual changes in the composition of influenza vaccines are necessary. Antigenic drift in the NA can lead to resistance against neuraminidase inhibitors.
Antigenic drift may also allow a virus to cross the species barrier to a new host. Of greater public health concern is the process of antigenic shift — also called reassortment — through which at least two different viruses combine, resulting in exchange of the HA for example H3 replaced by H5 and consequently the formation of a mosaic virus. Type A viruses undergo both antigenic drift and shift and are the only flu viruses known to cause pandemics, while flu type B viruses change only by the more gradual process of antigenic drift.
To receive weekly email updates about Seasonal Flu, enter your email address:. Skip directly to site content Skip directly to page options Skip directly to A-Z link. Influenza Flu. Section Navigation. Facebook Twitter LinkedIn Syndicate. Minus Related Pages. Influenza A viruses are constantly changing, making it possible on very rare occasions for non-human influenza viruses to change in such a way that they can infect people easily and spread efficiently from person to person.
Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the hemagglutinin H and the neuraminidase N.
There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes H1 through H18 and N1 through N Theoretically, any combination of the 18 hemagglutinins and 11 neuraminidase proteins are possible, but not all have been found in animals and even fewer have been found to infect humans. Antigenic shift represents an abrupt, major change in an influenza A virus. This can result from direct infection of humans with a non-human influenza A virus, such as a virus circulating among birds or pigs.
Antigenic shift also can happen when a non-human influenza A virus for example an avian influenza virus exchanges genetic information with other influenza A viruses in a process called genetic reassortment, and the resultant new virus is able to infect people. For example, an exchange of genes between a human influenza A virus and an avian influenza A virus can create a new influenza A virus with a hemagglutinin protein or both a hemagglutinin protein and a neuraminidase protein from an avian influenza A virus.
If this new virus causes illness in infected people and can spread easily from person to person , an influenza pandemic can occur. When a pandemic influenza virus emerges, the virus can spread quickly because most people will not be immune and a vaccine might not be widely available to offer immediate protection. As a pandemic influenza virus spreads, large numbers of people may need medical care worldwide. Schools, childcare centers, workplaces, and other places for mass gatherings may experience more absenteeism.
Public health and healthcare systems can become overloaded, with elevated rates of hospitalizations and deaths. Other critical infrastructure, such as law enforcement, emergency medical services, and transportation industry may also be affected. It is unlikely that seasonal flu vaccines would protect against a pandemic influenza virus.
Seasonal flu vaccines that are used annually protect against currently circulating human influenza A and B viruses. They are not designed to protect against new influenza A viruses. Disclosure of Potential Conflicts of Interest No potential conflicts of interest were disclosed. References 1. Rise of the microbes. Genesis of a highly pathogenic and potentially pandemic H5N1 influenza virus in eastern Asia. Evolution and ecology of influenza A viruses. Characterization of H1N1 swine influenza viruses circulating in Canadian pigs in Human infection with a novel avian-origin influenza A H7N9 virus.
Pu J, et al. Evolution of the H9N2 influenza genotype that facilitated the genesis of the novel H7N9 virus.
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