Novel Influenza (Flu) A Viruses

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Types of Influenza Viruses

There are four types of influenza (flu) viruses: A, B, C, and D. Influenza A and B viruses cause seasonal flu illness every fall and winter.

Influenza A viruses are divided into subtypes based on the two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11). Different animals can carry or be infected with influenza A viruses. Wild aquatic birds, such as ducks, have been identified to naturally circulate many of these subtype combinations. Current subtypes of influenza A viruses that routinely circulate in people include A(H1N1) and A(H3N2).

Influenza B viruses are classified into lineages: B/Yamagata and B/Victoria. In recent years, flu B/Yamagata has not been detected and has been removed from the 2024-2025 seasonal flu vaccine composition. Influenza B viruses generally change more slowly than influenza A viruses.

Influenza C virus infections cause mild illness in people and do not cause human epidemics.

Influenza D viruses mainly infect cattle with spillover to other animals, but are not known to infect people. 

How Influenza Viruses Change

Influenza (flu) viruses constantly change. The changes can happen slowly over time or suddenly.

Slow changes are called antigenic drift. Every flu season, usually from October to May in the United States, different types of seasonal influenza A and B viruses are passed from person-to-person, causing illness. Drift consists of small changes (or mutations) in the genes of influenza viruses. The slow and constant changes to the flu virus are the reason people need to get a flu vaccine each year. More information about seasonal flu and how to prevent it can be found on the VDH Flu Factsheet.

Sudden changes to flu viruses are called antigenic shift. Shift is an abrupt, major change in a flu A virus, resulting in new (or “novel) flu viruses that infect humans. This can occur when the genetic material from two flu viruses combine to form a virus with a new subtype or a mix of genes (including some from animals). Most people do not have immunity (protection) from novel flu viruses because they are different than seasonal flu. An example of this occurred in the spring of 2009 when a new H1N1 virus with a different combination of genes (from pigs, birds, and humans) emerged in people and quickly spread.

In summary, flu viruses undergo antigenic drift all the time, while antigenic shift happens infrequently.

Other Kinds of Influenza A Viruses 

Novel, avian, and variant influenza A viruses are different viruses from seasonal human influenza A viruses that circulate among people. These viruses are found in animals, including ducks, chickens (poultry), pigs (swine), cattle, whales, horses, seals, and cats. While it is unusual for people to get influenza infections directly from animals, some human infections and outbreaks caused by influenza A viruses have occurred after contact with infected animals. Some novel influenza A viruses are believed to pose a greater pandemic threat than others.

  • Novel influenza viruses can originate in animals where they gain the ability to infect and spread to or among humans. They can also originate from a human flu virus that changes significantly from the current seasonal flu A viruses.
  • Avian influenza or “bird flu” normally circulates in wild aquatic birds and can infect domestic poultry and other bird and animal species. Avian influenza viruses that have infected people are considered to be novel influenza A viruses.
  • Variant influenza viruses occur when people are infected with swine flu viruses, such as H1N2v and H1N3v. Swine flu is a respiratory disease of pigs that regularly causes outbreaks of flu in pigs.

Human infections with other kinds of influenza A viruses are uncommon. However,  human infections and outbreaks have occurred. These have usually occurred after unprotected exposure to sick or dead infected poultry, cattle, swine, or virus-contaminated environments. People with close or prolonged, unprotected exposures to infected birds or other animals (including livestock) or to environments contaminated by infected birds or other animals, are at greater risk of infection.

Last Updated: September 11, 2024