Influenza viruses are constantly evolving, in fact all influenza viruses undergo genetic changes over time. They can change in two different ways. One way they change is called “antigenic drift.” These are small changes in the genes of influenza viruses that happen continually over time as the virus replicates. When this happens, the body’s immune system may not recognize those viruses. The other type of change is called “antigenic shift.” Antigenic shift is an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin and/or new hemagglutinin and neuraminidase proteins in influenza viruses that infect humans. Shift results in a new influenza A subtype or a virus with a hemagglutinin or a hemagglutinin and neuraminidase combination that has emerged from an animal population that is so different from the same subtype in humans that most people do not have immunity to the new (e.g. novel) virus. Such a “shift” occurred in the spring of 2009, when an H1N1 virus with a new combination of genes emerged to infect people and quickly spread, causing a pandemic. When shift happens, most people have little or no protection against the new virus. While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.
The information CDC collects from studying genetic changes (also known as “substitutions,” “variants” or “mutations”) in influenza viruses plays an important public health role by helping to determine whether existing vaccines and medical countermeasures (e.g., antiviral drugs) will work against new influenza viruses, as well as helping to determine the potential for influenza viruses in animals to infect humans.
- Virus as vectors for human gene therapy
- Reassortment and reverse genetics
- Mutations of Influenza virus: Antigenic drift and shift
- Population genetics of Influenza virus
- Genome sequencing and genetic characterization